11 LECTURE: Working With Dates and Times
Optional Video
R has developed a special representation for dates and times. Dates are represented by the Date
class and times are represented by the POSIXct
or the POSIXlt
class. Dates are stored internally as the number of days since 1970-01-01 while times are stored internally as the number of seconds since 1970-01-01.
It’s not important to know the internal representation of dates and times in order to use them in R. I just thought those were fun facts.
11.1 Dates in R
Dates are represented by the Date
class and can be coerced from a character string using the as.Date()
function. This is a common way to end up with a Date
object in R.
## [1] "1970-01-01"
You can see the internal representation of a Date
object by using the unclass()
function.
## [1] 0
## [1] 1
11.2 Times in R
Optional Video
Times are represented by the POSIXct
or the POSIXlt
class. POSIXct
is just a very large integer under the hood. It use a useful class when you want to store times in something like a data frame. POSIXlt
is a list underneath and it stores a bunch of other useful information like the day of the week, day of the year, month, day of the month. This is useful when you need that kind of information.
There are a number of generic functions that work on dates and times to help you extract pieces of dates and/or times.
weekdays
: give the day of the weekmonths
: give the month namequarters
: give the quarter number (“Q1”, “Q2”, “Q3”, or “Q4”)
Times can be coerced from a character string using the as.POSIXlt
or as.POSIXct
function.
## [1] "2020-08-27 11:15:05 EDT"
## [1] "POSIXct" "POSIXt"
The POSIXlt
object contains some useful metadata.
## [1] "sec" "min" "hour" "mday" "mon" "year" "wday" "yday"
## [9] "isdst" "zone" "gmtoff"
## [1] 4
You can also use the POSIXct
format.
## [1] "2020-08-27 11:15:05 EDT"
## [1] 1598541305
## Error in x$sec: $ operator is invalid for atomic vectors
## [1] 5.319909
Finally, there is the strptime()
function in case your dates are
written in a different format. strptime()
takes a character vector that has dates and times and converts them into to a POSIXlt
object.
datestring <- c("January 10, 2012 10:40", "December 9, 2011 9:10")
x <- strptime(datestring, "%B %d, %Y %H:%M")
x
## [1] "2012-01-10 10:40:00 EST" "2011-12-09 09:10:00 EST"
## [1] "POSIXlt" "POSIXt"
The weird-looking symbols that start with the %
symbol are the formatting strings for dates and times. I can never remember the formatting strings. Check ?strptime
for details. It’s probably not worth memorizing this stuff.
11.3 Operations on Dates and Times
Optional Video
You can use mathematical operations on dates and times. Well, really just + and -. You can do comparisons too (i.e. ==, <=)
## Warning: Incompatible methods ("-.Date", "-.POSIXt") for "-"
## Error in x - y: non-numeric argument to binary operator
## Time difference of 356.3095 days
The nice thing about the date/time classes is that they keep track of all the annoying things about dates and times, like leap years, leap seconds, daylight savings, and time zones.
Here’s an example where a leap year gets involved.
## Time difference of 2 days
Here’s an example where two different time zones are in play (unless you live in GMT timezone, in which case they will be the same!).
## My local time zone
x <- as.POSIXct("2012-10-25 01:00:00")
y <- as.POSIXct("2012-10-25 06:00:00", tz = "GMT")
y-x
## Time difference of 1 hours
11.4 Summary
- Dates and times have special classes in R that allow for numerical and statistical calculations
- Dates use the
Date
class - Times use the
POSIXct
andPOSIXlt
class - Character strings can be coerced to Date/Time classes using the
strptime
function or theas.Date
,as.POSIXlt
, oras.POSIXct