The Date class

Remarks#

Jumbled notes

Date: Stores time as number of days since UNIX epoch on 1970-01-01. with negative values for earlier dates.
It is represented as an integer (however, it is not enforced in the internal representation)
They are always printed following the rules of the current Gregorian calendar, even though the calendar was not in use a long time ago.
It doesn’t keep track of timezones, so it should not be used to truncate the time out of POSIXct or POSIXlt objects.
sys.Date() returns an object of class Date

More notes

lubridate’s ymd, mdy, etc. are alternatives to as.Date that also parse to Date class; see Parsing dates and datetimes from strings with lubridate.
data.table’s experimental IDate class is derived from and is mostly interchangeable with Date, but is stored as integer instead of double.

Formatting Dates

To format Dates we use the format(date, format="%Y-%m-%d") function with either the POSIXct (given from as.POSIXct()) or POSIXlt (given from as.POSIXlt())

d = as.Date("2016-07-21") # Current Date Time Stamp

format(d,"%a")            # Abbreviated Weekday
## [1] "Thu"

format(d,"%A")            # Full Weekday
## [1] "Thursday"

format(d,"%b")            # Abbreviated Month
## [1] "Jul"

format(d,"%B")            # Full Month
## [1] "July"

format(d,"%m")            # 00-12 Month Format
## [1] "07"

format(d,"%d")            # 00-31 Day Format
## [1] "21"

format(d,"%e")            # 0-31 Day Format
## [1] "21"

format(d,"%y")            # 00-99 Year
## [1] "16"

format(d,"%Y")            # Year with Century
## [1] "2016"

For more, see ?strptime.

Dates

To coerce a variable to a date use the as.Date() function.

> x <- as.Date("2016-8-23")
> x
[1] "2016-08-23"
> class(x)
[1] "Date"

The as.Date() function allows you to provide a format argument. The default is %Y-%m-%d, which is Year-month-day.

> as.Date("23-8-2016", format="%d-%m-%Y") # To read in an European-style date
[1] "2016-08-23"

The format string can be placed either within a pair of single quotes or double quotes. Dates are usually expressed in a variety of forms such as: "d-m-yy" or "d-m-YYYY" or "m-d-yy" or "m-d-YYYY" or "YYYY-m-d" or "YYYY-d-m". These formats can also be expressed by replacing "-" by "/". Furher, dates are also expressed in the forms, say, “Nov 6, 1986” or “November 6, 1986” or “6 Nov, 1986” or “6 November, 1986” and so on. The as.Date() function accepts all such character strings and when we mention the appropriate format of the string, it always outputs the date in the form "YYYY-m-d".

Suppose we have a date string "9-6-1962" in the format "%d-%m-%Y".

#
# It tries to interprets the string as YYYY-m-d
#
> as.Date("9-6-1962")
[1] "0009-06-19"       #interprets as "%Y-%m-%d"
> 
as.Date("9/6/1962")
[1] "0009-06-19"       #again interprets as "%Y-%m-%d"
>
# It has no problem in understanding, if the date is in form  YYYY-m-d or YYYY/m/d
#
> as.Date("1962-6-9")
[1] "1962-06-09"        # no problem
> as.Date("1962/6/9")
[1] "1962-06-09"        # no problem
>

By specifying the correct format of the input string, we can get the desired results. We use the following codes for specifying the formats to the as.Date() function.

Format Code	Meaning
`%d`	day
`%m`	month
`%y`	year in 2-digits
`%Y`	year in 4-digits
`%b`	abbreviated month in 3 chars
`%B`	full name of the month

Consider the following example specifying the format parameter:

> as.Date("9-6-1962",format="%d-%m-%Y")
[1] "1962-06-09"
>

The parameter name format can be omitted.

> as.Date("9-6-1962", "%d-%m-%Y")
[1] "1962-06-09"
>

Some times, names of the months abbreviated to the first three characters are used in the writing the dates. In which case we use the format specifier %b.

> as.Date("6Nov1962","%d%b%Y")
[1] "1962-11-06"
>

Note that, there are no either '-' or '/' or white spaces between the members in the date string. The format string should exactly match that input string. Consider the following example:

> as.Date("6 Nov, 1962","%d %b, %Y")
[1] "1962-11-06"
>

Note that, there is a comma in the date string and hence a comma in the format specification too. If comma is omitted in the format string, it results in an NA. An example usage of %B format specifier is as follows:

> as.Date("October 12, 2016", "%B %d, %Y")
[1] "2016-10-12"
>
> as.Date("12 October, 2016", "%d %B, %Y")
[1] "2016-10-12"
>

%y format is system specific and hence, should be used with caution. Other parameters used with this function are origin and tz( time zone).

Parsing Strings into Date Objects

R contains a Date class, which is created with as.Date(), which takes a string or vector of strings, and if the date is not in ISO 8601 date format YYYY-MM-DD, a formatting string of strptime-style tokens.

as.Date('2016-08-01')    # in ISO format, so does not require formatting string
## [1] "2016-08-01"

as.Date('05/23/16', format = '%m/%d/%y')
## [1] "2016-05-23"

as.Date('March 23rd, 2016', '%B %drd, %Y')    # add separators and literals to format
## [1] "2016-03-23"

as.Date('  2016-08-01  foo')    # leading whitespace and all trailing characters are ignored
## [1] "2016-08-01"

as.Date(c('2016-01-01', '2016-01-02'))
# [1] "2016-01-01" "2016-01-02"