The Date Class
R has a special representation of date and time objects, dates are represented by the Date class, times and intervals by the POSIXct or POSIXlt classes.
Dates are stored internally as the number of days since 1/1/1970 while times and intervals are stored internally as the number of seconds elapsed since 1/1/1970
Classes for Times and Intervals
Dates are represented by the Date class and can be coerced (known as cast in other languages) by a string of characters as follows
x <- as.Date("1970-01-01")
x
[1] "1970-01-01"
> class(x)
[1] "Date"
Times and time intervals are represented using the classes
POSIXct and POSIXlt classes
-
POSIXctunder the “bonnet” is just a large integer and is used when you want to store times in something like a data frame -
POSIXlthas “detro” a list and stores a variety of other useful information such as day of the week, day of the year , month (and is most useful when you need to do manipulations)
There are a number of generic functions for date and time type objects
weekdays: returns the day of the weekmonths: returns the name of the monthquarters: returns the number of the quarter (“Q1”, “Q2”, “Q3”, “Q4”)
times can be obtained from a character string using the functions as.POSIXlt or as.POSIXct.
t1 <- Sys.time()
posix1<- as.POSIXlt(t1)
posix1$sec # takes the seconds part, I can do this because I built it as POSIXlt
or I can use the POSIXct format.
if dates are written in a different format, the strptime function can be used
datestring <- c("January 10, 2012 10:40", "December 9, 2011 9:10")
x <- strptime(datestring, "%B %d, %Y %H:%M")
#returns "2012-01-10 10:40:00 EST" "2011-12-09 09:10:00 EST" and interestingly I applied it to a list
Operations on Date and POSIX* type objects
You can use the mathematical operations + and - on dates and times as well as comparison operations such as == and <=
lct <- Sys.getlocale("LC_TIME"); Sys.setlocale("LC_TIME", "C")
x <- as.Date("2012-01-01");y <- strptime("January 10, 2012 10:40", "%B %d, %Y %H:%M")
x-y
Error in x - y : non-numeric argument transformed into binary operator
Also: Warning message:
Incompatible methods ("-.Date", "-.POSIXt") for "-"
x <- as.POSIXlt(x) # the types were incompatible I transform everything to POSIXlt
x-y
Time difference of -9.402778 days
# unclass also tells us that the only attribute we can draw on is the one we have already obtained, namely the difference in days
> unclass(difference)
[1] -9.402778
attr(, "units")
[1] "days"
# as anticipated I can also apply the sorting operators
x>y
[1] FALSE
also keeps track of leap years, daylight saving time and time zones.
> x <- as.Date("2012-03-01")
> y <- as.Date("2012-02-28")
> x-y
Time difference of 2 days
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 hour