Date Values

Dates are represented as the number of days since 1970-01-01, with negative values for earlier dates.

# use as.Date( ) to convert strings to dates
mydates <- as.Date(c("2007-06-22", "2004-02-13"))
# number of days between 6/22/07 and 2/13/04
days <- mydates[1] - mydates[2]

Sys.Date( ) returns today's date.
date() returns the current date and time.

The following symbols can be used with the format( ) function to print dates.

Symbol Meaning Example
%d day as a number (0-31) 01-31
%a abbreviated weekday Mon
%A unabbreviated weekday Monday
%m month (00-12) 00-12
%b abbreviated month Jan
%B unabbreviated month January
%y 2-digit year 07
%Y 4-digit year 2007

Here is an example.

# print today's date
today <-
format(today, format="%B %d %Y")
"June 20 2007"

Date Conversion

Character to Date

You can use the as.Date( ) function to convert character data to dates. The format is as.Date(x , "format"), where x is the character data and format gives the appropriate format.

# convert date info in format 'mm/dd/yyyy'
strDates <- c("01/05/1965", "08/16/1975")
dates <- as.Date(strDates, "%m/%d/%Y")

The default format is yyyy-mm-dd

mydates <- as.Date(c("2007-06-22", "2004-02-13"))

Date to Character

You can convert dates to character data using the as.Character( ) function.

# convert dates to character data
strDates <- as.character(dates)

Learning More

See help(as.Date) and help(strftime) for details on converting character data to dates. See help(ISOdatetime) for more information about formatting date/times.

To Practice

This intermediate R course includes a section on working with times and dates.