Dates and Times

.title[
# Dates and Times
]
.author[
### Luke Tierney
]
.institute[
### University of Iowa
]
.date[
### 2023-05-05
]

---

## Background
---

Data are often associated with a point in time, a particular

* year;
--

* month;
--

* day;
--

* hour, minute, second, ...

Some issues with points in time:

* Leap years and leap seconds.
--

* Daylight saving time.
--

* Local time or
  [_Coordinated Universal Time
      (UTC)_](https://www.timeanddate.com/time/aboututc.html).
--

* For historical data: changes in calendars.

---

R has data types to represent

* a particular date (`Date`);
--

* a particular second (`POSIXct`, date-time).

R stores dates as days since January 1, 1970, and date-times as the
the number of seconds since midnight on that day in
[_Coordinated Universal Time_
    (UTC)](https://www.timeanddate.com/time/aboututc.html).

Date objects are less complicated than data-time objects, so if you
only need dates you should stick with date objects.

Base R provides many facilities for dealing with dates and date-times.

The `lubridate` package provides a useful interface.

```r
library(lubridate)
```

The
[_Dates and Times_ chapter](https://r4ds.had.co.nz/dates-and-times.html)
of [_R for Data Science_](https://r4ds.had.co.nz/) provides more
details.

---
layout: true
## Creating Dates and Times
---

### Today and Now

.pull-left.code-80[
The `lubridate` function `today()` returns today's date as a `Date` object:

```r
today()
## [1] "2023-05-05"
```
{{content}}
]
--

```r
class(today())
## [1] "Date"
```
{{content}}
--

The `lubridate` function `now()` returns the current date-time as a
`POSIXct` object:

```r
now()
## [1] "2023-05-05 15:52:56 CDT"
```
{{content}}
--

```r
class(now())
## [1] "POSIXct" "POSIXt"
```

.pull-right.code-80[
The printed representation follows the international standard for the
representation of dates and times
([ISO8601](https://en.wikipedia.org/wiki/ISO_8601)).
{{content}}
]
--

Date and date-time objects can be used with addition and subtraction:
{{content}}

```r
now() + 3600  ## one hour from now
## [1] "2023-05-05 16:52:56 CDT"
```
{{content}}
--

```r
today() - 7   ## one week ago
## [1] "2023-04-28"
```

---

### Parsing Dates and Times From Strings

.pull-left.code-80[
Some common date formats:

```r
d1 <- "2023-04-16"
d2 <- "April 16, 2023"
d3 <- "16 April 2023"
d4 <- "16 April 23"
```
]

.pull-right.code-80[
These can be decoded by the functions `ymd()`, `mdy()`, and `dmy()`:
{{content}}
]
--

```r
ymd(d1)
## [1] "2023-04-16"
```
{{content}}
--

```r
mdy(d2)
## [1] "2023-04-16"
```
{{content}}
--

```r
dmy(d3)
## [1] "2023-04-16"
```
{{content}}
--

```r
dmy(d4)
## [1] "2023-04-16"
```

---

.pull-left.code-80[
By default, these functions use the current _locale_ settings for
interpreting month names or abbreviations.
{{content}}
]
--

```r
Sys.getlocale("LC_TIME")
## [1] "en_US.UTF-8"
```
{{content}}
--

If you need to parse a French date you might use

```r
dmy("16 Avril, 2023", locale = "fr_FR.UTF-8")
## [1] "2023-04-16"
```

.pull-right.code-80[
Date-times can be decoded with functions like `mdy_hm`:

```r
mdy_hm("April 16, 2023, 6:15 PM")
## [1] "2023-04-16 18:15:00 UTC"
```
{{content}}
]
--

```r
mdy_hms("April 16, 2023, 6:15:08 PM")
## [1] "2023-04-16 18:15:08 UTC"
```
{{content}}
--

By default these assume the time is specified in the UTC time zone.

---

.pull-left.code-80[
### Creating Dates and Times from Components

Dates can be created from year, month, and day by `make_date()`:
{{content}}
]
--

```r
make_date(2023, 4, 16)
## [1] "2023-04-16"
```
{{content}}

Creating a `date` variable from the `year`, `month`, and `day`
variables in the New York City `flights` table:

```r
library(nycflights13)
fl <- mutate(flights,
             date = make_date(year, month, day))
```
{{content}}
--

`ggplot` and other graphics systems know how to make useful axis
labels for dates:

```r
ggplot(count(fl, date)) +
    geom_line(aes(x = date, y = n))
```

<img src="datetime_files/figure-html/unnamed-chunk-19-1.png" style="display: block; margin: auto;" />
]
{{content}}
]
--

Weekday/weekend differenes are clearly visible.

---

.pull-left.code-80[
Date-times can be created from year, month, day, hour, minute, and second
using `make_datetime()`:
{{content}}
]
--

```r
make_datetime(2023, 4, 16, 18, 15)
## [1] "2023-04-16 18:15:00 UTC"
```
{{content}}
--

An attempt to recreate the `time_hour` variable in the flights table:

```r
fl <- mutate(fl,
             th = make_datetime(year, month, day,
                                hour))
```
{{content}}
--

This does not quite re-create the `time_hour` variable:

```r
identical(fl$th, fl$time_hour)
## [1] FALSE
```

.pull-right.code-80[

```r
fl$th[1]
## [1] "2013-01-01 05:00:00 UTC"
fl$time_hour[1]
## [1] "2013-01-01 05:00:00 EST"
```
{{content}}
]
--

By default, `make_datetime()` assumes the time points it is given are in UTC.

{{content}}
--

The `time_hour` variable is using local (eastern US) time.

{{content}}
--

We will look at time zones more [below](#time-zones).

---
layout: true
## Date and Time Components
---

Components of dates and date-times can be extracted with:

* `year()`, `month()`, `day()`, `hour()`, `minute()`, `second()`
* `yday()` -- day of the year
* `mday()` -- day of the month, same as `day`
* `wday()` -- day of the week

By default, `wday()` returns an integer:

```r
wday(today())
## [1] 6
```
]

But it can also return a label:

```r
wday(today(), label = TRUE)
## [1] Fri
## Levels: Sun < Mon < Tue < Wed < Thu < Fri < Sat
```
]

```r
wday(today(), label = TRUE, abbr = FALSE)
## [1] Friday
## 7 Levels: Sunday < Monday < Tuesday < Wednesday < Thursday < ... < Saturday
```
]

---

Weekday names and abbreviations are obviously locale-specific, and you
can specify an alternative to the default current locale:

```r
wday(today(), label = TRUE, abbr = FALSE, locale = "de_DE.UTF-8")
## [1] Freitag
## 7 Levels: Sonntag < Montag < Dienstag < Mittwoch < Donnerstag < ... < Samstag
```

```r
wday(today(), label = TRUE, locale = "de_DE.UTF-8")
## [1] Fr
## Levels: So < Mo < Di < Mi < Do < Fr < Sa
```
--

Even the integer value can be tricky:

* In the US, Canada, Japan the first day of the week is Sunday.

* In Germany, France, and the ISO8601 standard the first day of the
  week is Monday.

`wday()` can be asked to use a different first day, and a global default
can be set.

---

.pull-left.code-80[
Using `wday()` and the `date` variable we can look at the distribution
of the number of flights by day of the week:
{{content}}
]
--

```r
ggplot(fl, aes(x = wday(date, label = TRUE))) +
    geom_bar(fill = "deepskyblue3")
```

.pull-right[
<img src="datetime_files/figure-html/flights-wday-1.png" style="display: block; margin: auto;" />
{{content}}
]
--

There were substantially fewer flights on Saturdays but only slightly
fewer flights on Sundays.

---
layout: false
## Rounding

.pull-left[
`floor_date()`, `round_date()`, and `ceiling_date()` can be used to
round to a particular unit; the most useful are `week` and `quarter`.
{{content}}
]
--

Flights by week:

```r
as.character(wday(ymd("2013-01-01"),
                  label = TRUE, abbr = FALSE))
## [1] "Tuesday"
```

```r
as.character(wday(ymd("2013-12-31"),
                  label = TRUE, abbr = FALSE))
## [1] "Tuesday"
```
]

---
layout: true
## Time Spans
---

Subtracting dates or date-times produces `difftime` objects:

```r
now() - as_datetime(today())
## Time difference of 20.88339 hours
```

```r
today() - ymd("2023-01-01")
## Time difference of 124 days
```

Working with different units can be awkward; `lubridate` provides
_durations_, which always work in seconds:

```r
as.duration(now() - as_datetime(today()))
## [1] "75180.1998090744s (~20.88 hours)"
```

```r
as.duration(today() - ymd("2023-01-01"))
## [1] "10713600s (~17.71 weeks)"
```

---

Durations can be created with `dyears()`, `ddays()`, `dweeks()`, etc.:

```r
dyears(1)
## [1] "31557600s (~1 years)"
```

```r
ddays(1)
## [1] "86400s (~1 days)"
```

Durations can be added to a date or date-time object and can be
multiplied by a number:

```r
today()
## [1] "2023-05-05"
```
{{content}}
]
--

```r
today() + ddays(2)
## [1] "2023-05-07"
```
{{content}}
--

```r
today() + 2 * ddays(1)
## [1] "2023-05-07"
```

```r
(n1 <- now())
## [1] "2023-05-05 15:53:00 CDT"
```
{{content}}
]
--

```r
n1 + dminutes(3)
## [1] "2023-05-05 15:56:00 CDT"
```

---

Duations represent an exact number of seconds, which can lead to
surprises when DST is involved.

In 2023 the switch to DST happened in the US on March 12:

```r
ymd_hm("2023-03-11 23:02", tz = "America/Chicago") + ddays(1)
## [1] "2023-03-13 00:02:00 CDT"
```

_Periods_ are an alternative that may work more intuitively.

Periods are constructed with `years()`, `months()`, `days()`, etc:

```r
ymd_hm("2023-03-11 23:02", tz = "America/Chicago") + days(1)
## [1] "2023-03-12 23:02:00 CDT"
```

---
layout: true
## Time Zones
---

Date-time objects specify a point in time relative to second zero,
minute zero, hour zero, on January 1, 1970 in
[Coordinated Universal Time (UTC)](https://www.timeanddate.com/time/aboututc.html).

Date-time objects can have a time zone associated with them that
affects how they are printed.

`now()` returns a date-time object with the time zone set as the
local time zone of the computer.

```r
now()
## [1] "2023-05-05 15:53:00 CDT"
```

Time zones are complex, they can change on a regular basis (DST) or as
a result of politics.

When a date-time object is created from components, by default it is
given the UTC time zone.

---

To create a point in time based on local time information, such as 10
AM on April 16, 2023, in Iowa City, a time zone for interpreting the
local time needs to be specified.

The short notations like CDT are not adequate for this: Both the US
 and Australia have EST, which are quite different.

R uses the
[_Internet Assigned Numbers Authority_ (IANA)](https://www.iana.org/time-zones)
naming convention and data base.

The local time zone is:

```r
Sys.timezone()
## [1] "America/Chicago"
```

The time point 10:00:00 AM on April 16, 2023 in Iowa City can be
specified as

```r
(tm <- make_datetime(2023, 4, 16, 10, tz = "America/Chicago"))
## [1] "2023-04-16 10:00:00 CDT"
```

---

Time zones of date-time objects can be changed in two ways:

* `with_tz` keeps the instant in time and changes the time zone used
  for display.

* `force_tz` changes the instant in time; use this if the time zone is
  incorrectly specified but the clock time is correct.

The available time zone specifications are contained in `OlsonNames`:

```r
OlsonNames()
##   [1] "Africa/Abidjan"                   "Africa/Accra"                    
##   [3] "Africa/Addis_Ababa"               "Africa/Algiers"                  
##   [5] "Africa/Asmara"                    "Africa/Asmera"                   
##   [7] "Africa/Bamako"                    "Africa/Bangui"                   
##   [9] "Africa/Banjul"                    "Africa/Bissau"                   
##  [11] "Africa/Blantyre"                  "Africa/Brazzaville"              
##  [13] "Africa/Bujumbura"                 "Africa/Cairo"                    
##  [15] "Africa/Casablanca"                "Africa/Ceuta"                    
##  [17] "Africa/Conakry"                   "Africa/Dakar"                    
##  [19] "Africa/Dar_es_Salaam"             "Africa/Djibouti"                 
##  [21] "Africa/Douala"                    "Africa/El_Aaiun"                 
##  [23] "Africa/Freetown"                  "Africa/Gaborone"                 
##  [25] "Africa/Harare"                    "Africa/Johannesburg"             
##  [27] "Africa/Juba"                      "Africa/Kampala"                  
##  [29] "Africa/Khartoum"                  "Africa/Kigali"                   
##  [31] "Africa/Kinshasa"                  "Africa/Lagos"                    
##  [33] "Africa/Libreville"                "Africa/Lome"                     
##  [35] "Africa/Luanda"                    "Africa/Lubumbashi"               
##  [37] "Africa/Lusaka"                    "Africa/Malabo"                   
##  [39] "Africa/Maputo"                    "Africa/Maseru"                   
##  [41] "Africa/Mbabane"                   "Africa/Mogadishu"                
##  [43] "Africa/Monrovia"                  "Africa/Nairobi"                  
##  [45] "Africa/Ndjamena"                  "Africa/Niamey"                   
##  [47] "Africa/Nouakchott"                "Africa/Ouagadougou"              
##  [49] "Africa/Porto-Novo"                "Africa/Sao_Tome"                 
##  [51] "Africa/Timbuktu"                  "Africa/Tripoli"                  
##  [53] "Africa/Tunis"                     "Africa/Windhoek"                 
##  [55] "America/Adak"                     "America/Anchorage"               
##  [57] "America/Anguilla"                 "America/Antigua"                 
##  [59] "America/Araguaina"                "America/Argentina/Buenos_Aires"  
##  [61] "America/Argentina/Catamarca"      "America/Argentina/ComodRivadavia"
##  [63] "America/Argentina/Cordoba"        "America/Argentina/Jujuy"         
##  [65] "America/Argentina/La_Rioja"       "America/Argentina/Mendoza"       
##  [67] "America/Argentina/Rio_Gallegos"   "America/Argentina/Salta"         
##  [69] "America/Argentina/San_Juan"       "America/Argentina/San_Luis"      
##  [71] "America/Argentina/Tucuman"        "America/Argentina/Ushuaia"       
##  [73] "America/Aruba"                    "America/Asuncion"                
##  [75] "America/Atikokan"                 "America/Atka"                    
##  [77] "America/Bahia"                    "America/Bahia_Banderas"          
##  [79] "America/Barbados"                 "America/Belem"                   
##  [81] "America/Belize"                   "America/Blanc-Sablon"            
##  [83] "America/Boa_Vista"                "America/Bogota"                  
##  [85] "America/Boise"                    "America/Buenos_Aires"            
##  [87] "America/Cambridge_Bay"            "America/Campo_Grande"            
##  [89] "America/Cancun"                   "America/Caracas"                 
##  [91] "America/Catamarca"                "America/Cayenne"                 
##  [93] "America/Cayman"                   "America/Chicago"                 
##  [95] "America/Chihuahua"                "America/Ciudad_Juarez"           
##  [97] "America/Coral_Harbour"            "America/Cordoba"                 
##  [99] "America/Costa_Rica"               "America/Creston"                 
## [101] "America/Cuiaba"                   "America/Curacao"                 
## [103] "America/Danmarkshavn"             "America/Dawson"                  
## [105] "America/Dawson_Creek"             "America/Denver"                  
## [107] "America/Detroit"                  "America/Dominica"                
## [109] "America/Edmonton"                 "America/Eirunepe"                
## [111] "America/El_Salvador"              "America/Ensenada"                
## [113] "America/Fort_Nelson"              "America/Fort_Wayne"              
## [115] "America/Fortaleza"                "America/Glace_Bay"               
## [117] "America/Godthab"                  "America/Goose_Bay"               
## [119] "America/Grand_Turk"               "America/Grenada"                 
## [121] "America/Guadeloupe"               "America/Guatemala"               
## [123] "America/Guayaquil"                "America/Guyana"                  
## [125] "America/Halifax"                  "America/Havana"                  
## [127] "America/Hermosillo"               "America/Indiana/Indianapolis"    
## [129] "America/Indiana/Knox"             "America/Indiana/Marengo"         
## [131] "America/Indiana/Petersburg"       "America/Indiana/Tell_City"       
## [133] "America/Indiana/Vevay"            "America/Indiana/Vincennes"       
## [135] "America/Indiana/Winamac"          "America/Indianapolis"            
## [137] "America/Inuvik"                   "America/Iqaluit"                 
## [139] "America/Jamaica"                  "America/Jujuy"                   
## [141] "America/Juneau"                   "America/Kentucky/Louisville"     
## [143] "America/Kentucky/Monticello"      "America/Knox_IN"                 
## [145] "America/Kralendijk"               "America/La_Paz"                  
## [147] "America/Lima"                     "America/Los_Angeles"             
## [149] "America/Louisville"               "America/Lower_Princes"           
## [151] "America/Maceio"                   "America/Managua"                 
## [153] "America/Manaus"                   "America/Marigot"                 
## [155] "America/Martinique"               "America/Matamoros"               
## [157] "America/Mazatlan"                 "America/Mendoza"                 
## [159] "America/Menominee"                "America/Merida"                  
## [161] "America/Metlakatla"               "America/Mexico_City"             
## [163] "America/Miquelon"                 "America/Moncton"                 
## [165] "America/Monterrey"                "America/Montevideo"              
## [167] "America/Montreal"                 "America/Montserrat"              
## [169] "America/Nassau"                   "America/New_York"                
## [171] "America/Nipigon"                  "America/Nome"                    
## [173] "America/Noronha"                  "America/North_Dakota/Beulah"     
## [175] "America/North_Dakota/Center"      "America/North_Dakota/New_Salem"  
## [177] "America/Nuuk"                     "America/Ojinaga"                 
## [179] "America/Panama"                   "America/Pangnirtung"             
## [181] "America/Paramaribo"               "America/Phoenix"                 
## [183] "America/Port_of_Spain"            "America/Port-au-Prince"          
## [185] "America/Porto_Acre"               "America/Porto_Velho"             
## [187] "America/Puerto_Rico"              "America/Punta_Arenas"            
## [189] "America/Rainy_River"              "America/Rankin_Inlet"            
## [191] "America/Recife"                   "America/Regina"                  
## [193] "America/Resolute"                 "America/Rio_Branco"              
## [195] "America/Rosario"                  "America/Santa_Isabel"            
## [197] "America/Santarem"                 "America/Santiago"                
## [199] "America/Santo_Domingo"            "America/Sao_Paulo"               
## [201] "America/Scoresbysund"             "America/Shiprock"                
## [203] "America/Sitka"                    "America/St_Barthelemy"           
## [205] "America/St_Johns"                 "America/St_Kitts"                
## [207] "America/St_Lucia"                 "America/St_Thomas"               
## [209] "America/St_Vincent"               "America/Swift_Current"           
## [211] "America/Tegucigalpa"              "America/Thule"                   
## [213] "America/Thunder_Bay"              "America/Tijuana"                 
## [215] "America/Toronto"                  "America/Tortola"                 
## [217] "America/Vancouver"                "America/Virgin"                  
## [219] "America/Whitehorse"               "America/Winnipeg"                
## [221] "America/Yakutat"                  "America/Yellowknife"             
## [223] "Antarctica/Casey"                 "Antarctica/Davis"                
## [225] "Antarctica/DumontDUrville"        "Antarctica/Macquarie"            
## [227] "Antarctica/Mawson"                "Antarctica/McMurdo"              
## [229] "Antarctica/Palmer"                "Antarctica/Rothera"              
## [231] "Antarctica/South_Pole"            "Antarctica/Syowa"                
## [233] "Antarctica/Troll"                 "Antarctica/Vostok"               
## [235] "Arctic/Longyearbyen"              "Asia/Aden"                       
## [237] "Asia/Almaty"                      "Asia/Amman"                      
## [239] "Asia/Anadyr"                      "Asia/Aqtau"                      
## [241] "Asia/Aqtobe"                      "Asia/Ashgabat"                   
## [243] "Asia/Ashkhabad"                   "Asia/Atyrau"                     
## [245] "Asia/Baghdad"                     "Asia/Bahrain"                    
## [247] "Asia/Baku"                        "Asia/Bangkok"                    
## [249] "Asia/Barnaul"                     "Asia/Beirut"                     
## [251] "Asia/Bishkek"                     "Asia/Brunei"                     
## [253] "Asia/Calcutta"                    "Asia/Chita"                      
## [255] "Asia/Choibalsan"                  "Asia/Chongqing"                  
## [257] "Asia/Chungking"                   "Asia/Colombo"                    
## [259] "Asia/Dacca"                       "Asia/Damascus"                   
## [261] "Asia/Dhaka"                       "Asia/Dili"                       
## [263] "Asia/Dubai"                       "Asia/Dushanbe"                   
## [265] "Asia/Famagusta"                   "Asia/Gaza"                       
## [267] "Asia/Hanoi"                       "Asia/Harbin"                     
## [269] "Asia/Hebron"                      "Asia/Ho_Chi_Minh"                
## [271] "Asia/Hong_Kong"                   "Asia/Hovd"                       
## [273] "Asia/Irkutsk"                     "Asia/Istanbul"                   
## [275] "Asia/Jakarta"                     "Asia/Jayapura"                   
## [277] "Asia/Jerusalem"                   "Asia/Kabul"                      
## [279] "Asia/Kamchatka"                   "Asia/Karachi"                    
## [281] "Asia/Kashgar"                     "Asia/Kathmandu"                  
## [283] "Asia/Katmandu"                    "Asia/Khandyga"                   
## [285] "Asia/Kolkata"                     "Asia/Krasnoyarsk"                
## [287] "Asia/Kuala_Lumpur"                "Asia/Kuching"                    
## [289] "Asia/Kuwait"                      "Asia/Macao"                      
## [291] "Asia/Macau"                       "Asia/Magadan"                    
## [293] "Asia/Makassar"                    "Asia/Manila"                     
## [295] "Asia/Muscat"                      "Asia/Nicosia"                    
## [297] "Asia/Novokuznetsk"                "Asia/Novosibirsk"                
## [299] "Asia/Omsk"                        "Asia/Oral"                       
## [301] "Asia/Phnom_Penh"                  "Asia/Pontianak"                  
## [303] "Asia/Pyongyang"                   "Asia/Qatar"                      
## [305] "Asia/Qostanay"                    "Asia/Qyzylorda"                  
## [307] "Asia/Rangoon"                     "Asia/Riyadh"                     
## [309] "Asia/Saigon"                      "Asia/Sakhalin"                   
## [311] "Asia/Samarkand"                   "Asia/Seoul"                      
## [313] "Asia/Shanghai"                    "Asia/Singapore"                  
## [315] "Asia/Srednekolymsk"               "Asia/Taipei"                     
## [317] "Asia/Tashkent"                    "Asia/Tbilisi"                    
## [319] "Asia/Tehran"                      "Asia/Tel_Aviv"                   
## [321] "Asia/Thimbu"                      "Asia/Thimphu"                    
## [323] "Asia/Tokyo"                       "Asia/Tomsk"                      
## [325] "Asia/Ujung_Pandang"               "Asia/Ulaanbaatar"                
## [327] "Asia/Ulan_Bator"                  "Asia/Urumqi"                     
## [329] "Asia/Ust-Nera"                    "Asia/Vientiane"                  
## [331] "Asia/Vladivostok"                 "Asia/Yakutsk"                    
## [333] "Asia/Yangon"                      "Asia/Yekaterinburg"              
## [335] "Asia/Yerevan"                     "Atlantic/Azores"                 
## [337] "Atlantic/Bermuda"                 "Atlantic/Canary"                 
## [339] "Atlantic/Cape_Verde"              "Atlantic/Faeroe"                 
## [341] "Atlantic/Faroe"                   "Atlantic/Jan_Mayen"              
## [343] "Atlantic/Madeira"                 "Atlantic/Reykjavik"              
## [345] "Atlantic/South_Georgia"           "Atlantic/St_Helena"              
## [347] "Atlantic/Stanley"                 "Australia/ACT"                   
## [349] "Australia/Adelaide"               "Australia/Brisbane"              
## [351] "Australia/Broken_Hill"            "Australia/Canberra"              
## [353] "Australia/Currie"                 "Australia/Darwin"                
## [355] "Australia/Eucla"                  "Australia/Hobart"                
## [357] "Australia/LHI"                    "Australia/Lindeman"              
## [359] "Australia/Lord_Howe"              "Australia/Melbourne"             
## [361] "Australia/North"                  "Australia/NSW"                   
## [363] "Australia/Perth"                  "Australia/Queensland"            
## [365] "Australia/South"                  "Australia/Sydney"                
## [367] "Australia/Tasmania"               "Australia/Victoria"              
## [369] "Australia/West"                   "Australia/Yancowinna"            
## [371] "Brazil/Acre"                      "Brazil/DeNoronha"                
## [373] "Brazil/East"                      "Brazil/West"                     
## [375] "Canada/Atlantic"                  "Canada/Central"                  
## [377] "Canada/Eastern"                   "Canada/Mountain"                 
## [379] "Canada/Newfoundland"              "Canada/Pacific"                  
## [381] "Canada/Saskatchewan"              "Canada/Yukon"                    
## [383] "CET"                              "Chile/Continental"               
## [385] "Chile/EasterIsland"               "CST6CDT"                         
## [387] "Cuba"                             "EET"                             
## [389] "Egypt"                            "Eire"                            
## [391] "EST"                              "EST5EDT"                         
## [393] "Etc/GMT"                          "Etc/GMT-0"                       
## [395] "Etc/GMT-1"                        "Etc/GMT-10"                      
## [397] "Etc/GMT-11"                       "Etc/GMT-12"                      
## [399] "Etc/GMT-13"                       "Etc/GMT-14"                      
## [401] "Etc/GMT-2"                        "Etc/GMT-3"                       
## [403] "Etc/GMT-4"                        "Etc/GMT-5"                       
## [405] "Etc/GMT-6"                        "Etc/GMT-7"                       
## [407] "Etc/GMT-8"                        "Etc/GMT-9"                       
## [409] "Etc/GMT+0"                        "Etc/GMT+1"                       
## [411] "Etc/GMT+10"                       "Etc/GMT+11"                      
## [413] "Etc/GMT+12"                       "Etc/GMT+2"                       
## [415] "Etc/GMT+3"                        "Etc/GMT+4"                       
## [417] "Etc/GMT+5"                        "Etc/GMT+6"                       
## [419] "Etc/GMT+7"                        "Etc/GMT+8"                       
## [421] "Etc/GMT+9"                        "Etc/GMT0"                        
## [423] "Etc/Greenwich"                    "Etc/UCT"                         
## [425] "Etc/Universal"                    "Etc/UTC"                         
## [427] "Etc/Zulu"                         "Europe/Amsterdam"                
## [429] "Europe/Andorra"                   "Europe/Astrakhan"                
## [431] "Europe/Athens"                    "Europe/Belfast"                  
## [433] "Europe/Belgrade"                  "Europe/Berlin"                   
## [435] "Europe/Bratislava"                "Europe/Brussels"                 
## [437] "Europe/Bucharest"                 "Europe/Budapest"                 
## [439] "Europe/Busingen"                  "Europe/Chisinau"                 
## [441] "Europe/Copenhagen"                "Europe/Dublin"                   
## [443] "Europe/Gibraltar"                 "Europe/Guernsey"                 
## [445] "Europe/Helsinki"                  "Europe/Isle_of_Man"              
## [447] "Europe/Istanbul"                  "Europe/Jersey"                   
## [449] "Europe/Kaliningrad"               "Europe/Kiev"                     
## [451] "Europe/Kirov"                     "Europe/Kyiv"                     
## [453] "Europe/Lisbon"                    "Europe/Ljubljana"                
## [455] "Europe/London"                    "Europe/Luxembourg"               
## [457] "Europe/Madrid"                    "Europe/Malta"                    
## [459] "Europe/Mariehamn"                 "Europe/Minsk"                    
## [461] "Europe/Monaco"                    "Europe/Moscow"                   
## [463] "Europe/Nicosia"                   "Europe/Oslo"                     
## [465] "Europe/Paris"                     "Europe/Podgorica"                
## [467] "Europe/Prague"                    "Europe/Riga"                     
## [469] "Europe/Rome"                      "Europe/Samara"                   
## [471] "Europe/San_Marino"                "Europe/Sarajevo"                 
## [473] "Europe/Saratov"                   "Europe/Simferopol"               
## [475] "Europe/Skopje"                    "Europe/Sofia"                    
## [477] "Europe/Stockholm"                 "Europe/Tallinn"                  
## [479] "Europe/Tirane"                    "Europe/Tiraspol"                 
## [481] "Europe/Ulyanovsk"                 "Europe/Uzhgorod"                 
## [483] "Europe/Vaduz"                     "Europe/Vatican"                  
## [485] "Europe/Vienna"                    "Europe/Vilnius"                  
## [487] "Europe/Volgograd"                 "Europe/Warsaw"                   
## [489] "Europe/Zagreb"                    "Europe/Zaporozhye"               
## [491] "Europe/Zurich"                    "Factory"                         
## [493] "GB"                               "GB-Eire"                         
## [495] "GMT"                              "GMT-0"                           
## [497] "GMT+0"                            "GMT0"                            
## [499] "Greenwich"                        "Hongkong"                        
## [501] "HST"                              "Iceland"                         
## [503] "Indian/Antananarivo"              "Indian/Chagos"                   
## [505] "Indian/Christmas"                 "Indian/Cocos"                    
## [507] "Indian/Comoro"                    "Indian/Kerguelen"                
## [509] "Indian/Mahe"                      "Indian/Maldives"                 
## [511] "Indian/Mauritius"                 "Indian/Mayotte"                  
## [513] "Indian/Reunion"                   "Iran"                            
## [515] "Israel"                           "Jamaica"                         
## [517] "Japan"                            "Kwajalein"                       
## [519] "Libya"                            "MET"                             
## [521] "Mexico/BajaNorte"                 "Mexico/BajaSur"                  
## [523] "Mexico/General"                   "MST"                             
## [525] "MST7MDT"                          "Navajo"                          
## [527] "NZ"                               "NZ-CHAT"                         
## [529] "Pacific/Apia"                     "Pacific/Auckland"                
## [531] "Pacific/Bougainville"             "Pacific/Chatham"                 
## [533] "Pacific/Chuuk"                    "Pacific/Easter"                  
## [535] "Pacific/Efate"                    "Pacific/Enderbury"               
## [537] "Pacific/Fakaofo"                  "Pacific/Fiji"                    
## [539] "Pacific/Funafuti"                 "Pacific/Galapagos"               
## [541] "Pacific/Gambier"                  "Pacific/Guadalcanal"             
## [543] "Pacific/Guam"                     "Pacific/Honolulu"                
## [545] "Pacific/Johnston"                 "Pacific/Kanton"                  
## [547] "Pacific/Kiritimati"               "Pacific/Kosrae"                  
## [549] "Pacific/Kwajalein"                "Pacific/Majuro"                  
## [551] "Pacific/Marquesas"                "Pacific/Midway"                  
## [553] "Pacific/Nauru"                    "Pacific/Niue"                    
## [555] "Pacific/Norfolk"                  "Pacific/Noumea"                  
## [557] "Pacific/Pago_Pago"                "Pacific/Palau"                   
## [559] "Pacific/Pitcairn"                 "Pacific/Pohnpei"                 
## [561] "Pacific/Ponape"                   "Pacific/Port_Moresby"            
## [563] "Pacific/Rarotonga"                "Pacific/Saipan"                  
## [565] "Pacific/Samoa"                    "Pacific/Tahiti"                  
## [567] "Pacific/Tarawa"                   "Pacific/Tongatapu"               
## [569] "Pacific/Truk"                     "Pacific/Wake"                    
## [571] "Pacific/Wallis"                   "Pacific/Yap"                     
## [573] "Poland"                           "Portugal"                        
## [575] "PRC"                              "PST8PDT"                         
## [577] "ROC"                              "ROK"                             
## [579] "Singapore"                        "Turkey"                          
## [581] "UCT"                              "Universal"                       
## [583] "US/Alaska"                        "US/Aleutian"                     
## [585] "US/Arizona"                       "US/Central"                      
## [587] "US/East-Indiana"                  "US/Eastern"                      
## [589] "US/Hawaii"                        "US/Indiana-Starke"               
## [591] "US/Michigan"                      "US/Mountain"                     
## [593] "US/Pacific"                       "US/Samoa"                        
## [595] "UTC"                              "W-SU"                            
## [597] "WET"                              "Zulu"                            
## attr(,"Version")
## [1] "2023c"
```
]

---

The instant `tm` in some other time zones:

```r
with_tz(tm, tz = "UTC")
## [1] "2023-04-16 15:00:00 UTC"
```
{{content}}

]
--

```r
with_tz(tm, tz = "America/New_York")
## [1] "2023-04-16 11:00:00 EDT"
```
{{content}}
--

```r
with_tz(tm, tz = "Asia/Shanghai")
## [1] "2023-04-16 23:00:00 CST"
```
{{content}}
--

```r
with_tz(tm, tz = "Pacific/Auckland")
## [1] "2023-04-17 03:00:00 NZST"
```

```r
with_tz(tm, tz = "Asia/Kolkata")
## [1] "2023-04-16 20:30:00 IST"
```
{{content}}
]
--

```r
with_tz(tm, tz = "Canada/Newfoundland")
## [1] "2023-04-16 12:30:00 NDT"
```
{{content}}
--

```r
with_tz(tm, tz = "Asia/Katmandu")
## [1] "2023-04-16 20:45:00 +0545"
```
---

Some more examples:

.pull-left.small-code.scroll-box-20[

```r
## All offsets that are not a full hour:
get_offset <- function(z)
    abs(minute(with_tz(tm, tz = z)) - minute(tm))
offsets <- data.frame(zone = OlsonNames()) %>%
    mutate(offset = sapply(zone, get_offset)) %>%
    arrange(offset)
filter(offsets, offset != 0)
##                     zone offset
## 1       America/St_Johns     30
## 2          Asia/Calcutta     30
## 3           Asia/Colombo     30
## 4             Asia/Kabul     30
## 5           Asia/Kolkata     30
## 6           Asia/Rangoon     30
## 7            Asia/Tehran     30
## 8            Asia/Yangon     30
## 9     Australia/Adelaide     30
## 10 Australia/Broken_Hill     30
## 11      Australia/Darwin     30
## 12         Australia/LHI     30
## 13   Australia/Lord_Howe     30
## 14       Australia/North     30
## 15       Australia/South     30
## 16  Australia/Yancowinna     30
## 17   Canada/Newfoundland     30
## 18          Indian/Cocos     30
## 19                  Iran     30
## 20     Pacific/Marquesas     30
## 21        Asia/Kathmandu     45
## 22         Asia/Katmandu     45
## 23       Australia/Eucla     45
## 24               NZ-CHAT     45
## 25       Pacific/Chatham     45
```
]

.pull-right.small-code.scroll-box-20[

```r
## Offsets for Australia:
filter(offsets, grepl("Australia", zone))
##                     zone offset
## 1          Australia/ACT      0
## 2     Australia/Brisbane      0
## 3     Australia/Canberra      0
## 4       Australia/Currie      0
## 5       Australia/Hobart      0
## 6     Australia/Lindeman      0
## 7    Australia/Melbourne      0
## 8          Australia/NSW      0
## 9        Australia/Perth      0
## 10  Australia/Queensland      0
## 11      Australia/Sydney      0
## 12    Australia/Tasmania      0
## 13    Australia/Victoria      0
## 14        Australia/West      0
## 15    Australia/Adelaide     30
## 16 Australia/Broken_Hill     30
## 17      Australia/Darwin     30
## 18         Australia/LHI     30
## 19   Australia/Lord_Howe     30
## 20       Australia/North     30
## 21       Australia/South     30
## 22  Australia/Yancowinna     30
## 23       Australia/Eucla     45
```
]

---

If we create the `th` variable for the flights data as

```r
fl <- mutate(flights, th = make_datetime(year, month, day, hour,
                                         tz = "America/New_York"))
```

then the result matches the `date_time` variable:

```r
identical(fl$th, fl$time_hour)
## [1] TRUE
```

---

The `time_hour` variable in the `weather` table reflects actual points
in time and, together with `origin`, can serve as a primary key:

```r
filter(count(weather, origin, time_hour), n > 1)
## # A tibble: 0 × 3
## # ℹ 3 variables: origin <chr>, time_hour <dttm>, n <int>
```

The `month`, `day`, `hour` variables are confused by the time
change.

.pull-left.code-80[
In November there is a repeat:

```r
count(weather, origin, month, day, hour) %>%
    filter(n > 1)
## # A tibble: 3 × 5
##   origin month   day  hour     n
##   <chr>  <int> <int> <int> <int>
## 1 EWR       11     3     1     2
## 2 JFK       11     3     1     2
## 3 LGA       11     3     1     2
```
]

.pull-right.code-80[
and there is a missing hour in March:

```r
select(weather, origin, month, day, hour) %>%
    filter(origin == "EWR", month == 3,
           day == 10, hour <= 3)
## # A tibble: 3 × 4
##   origin month   day  hour
##   <chr>  <int> <int> <int>
## 1 EWR        3    10     0
## 2 EWR        3    10     1
## 3 EWR        3    10     3
```
]

---
layout: false
## Things to Look Out For

For dates:

* Language used for months and weekdays, and their abbreviations.

* Ambiguous numerical conventions like 4/11/2023: is this April 11 or
  November 4?

* Day 2 of the week: is this Monday or Tuesday?

* For historical data, what calendar is being used? (The _October
  Revolution_ happened on November 6/7, 1917 by the current Gregorian
  calendar; October 24/25 by the Julian calendar Russia was still
  using.)

For date-times

* All of the above.

* Daylight saving time.

* Time zones.

<!--
library(lubridate)
flights <- mutate(flights, th = make_datetime(year, month, day, hour,
                                              tz = "America/New_York"))
weather <- mutate(weather, th = make_datetime(year, month, day, hour,
                                              tz = "UTC"))

fl0 <- select(filter(flights, origin == "LGA"),
              dep_time, time_hour, th)
w0 <- select(filter(weather, origin == "LGA")[-(1 : 9),],
              time_hour, th, temp)

fl1 <- left_join(fl0, select(w0, -th), "time_hour")
fl2 <- left_join(fl0, select(w0, -time_hour), "th")
-->

---
layout: false
## Reading

Chapter [_Dates and Times_](https://r4ds.had.co.nz/dates-and-times.html)
in [_R for Data Science_](https://r4ds.had.co.nz/).

---
layout: true
## Exercises
---

1) Using the NYC flights data, how many flights were there on
   Saturdays from Newark (EWR) to Cicago O'Hare (ORD) in 2013?
   
*   a. 413
*   b. 522
*   c. 601
*   d. 733
   
---

2) What day of the week will July 4, 2030, fall on?

*   a. Monday
*   b. Wednesday
*   d. Thursday
*   c. Saturday

//adapted from Emi Tanaka's gist at //https://gist.github.com/emitanaka/eaa258bb8471c041797ff377704c8505