Date and Time

Date and Time

Date is represented by Date class. Date is stored as number of days since 1970-01-01.

Time is represented by POSIXct (represents date as large integer) or POSIXlt (represents date as list of values) classes. Time is stored as number of seconds since 1970-01-01.

Dates

Get current time.

> date = date()
[1] "Mon Jan  5 10:54:17 2015"

> class(date)
[1] "character"

Get system date.

date = Sys.Date()
[1] "2015-01-05"

class(date)
[1] "Date"

Formatting and Parsing

We can use format function to format dates.

> date = Sys.Date()

> format(date, "%a %b %d")
[1] "Mon Jan 05"

> as.Date("1982-01-01")
[1] "1982-01-01"

time <- Sys.time()

as.POSIXlt(time)
as.POSIXct(time)

Create dates from vector

Have a look here for help with formatting strings.

> strings = c("05-01-2015", "06-01-2015")
> dates = as.Date(strings, "%d-%m-%Y")
> dates
[1] "2015-01-05" "2015-01-06"

After the strings are converted to Date type, we can, for example, easily find the difference in days.

> dates[2] - dates[1]
Time difference of 1 days

> as.numeric(dates[2] - dates[1])
[1] 1

Or, for example do the following.

> weekdays(date[1])
[1] "Monday"

> months(date[1])
[1] "January"

> julian(date[1])
[1] 16440
attr(,"origin")
[1] "1970-01-01"

lubridate package

We can use lubridate package to work with dates.

install.packages("lubridate")
library(lubridate)

Then we can use functions like:

> ymd("20150105")
[1] "2015-01-05 UTC"

> myd("05-2015-01")
[1] "2015-05-01 UTC"

> ymd_hms("2015-01-05 10:15:01")
[1] "2015-01-05 10:15:01 UTC"

> wday(ymd("20150105"))
[1] 2

More info about the package is here.