as.Date: Date Conversion Functions to and from Character
In robertzk/monadicbase: The R Base Package

Description Usage Arguments Details Value Conversion from other Systems Note References See Also Examples

Functions to convert between character representations and objects of class "Date" representing calendar dates.

as.Date(x, ...)
## S3 method for class 'character'
as.Date(x, format, ...)
## S3 method for class 'numeric'
as.Date(x, origin, ...)
## S3 method for class 'POSIXct'
as.Date(x, tz = "UTC", ...)

## S3 method for class 'Date'
format(x, ...)

## S3 method for class 'Date'
as.character(x, ...)

`x`	An object to be converted.
`format`	A character string. If not specified, it will try `"%Y-%m-%d"` then `"%Y/%m/%d"` on the first non-`NA` element, and give an error if neither works. Otherwise, the processing is via `strptime`
`origin`	a Date object, or something which can be coerced by `as.Date(origin, ...)` to such an object.
`tz`	a time zone name.
`...`	Further arguments to be passed from or to other methods, including `format` for `as.character` and `as.Date` methods.

The usual vector re-cycling rules are applied to x and format so the answer will be of length that of the longer of the vectors.

Locale-specific conversions to and from character strings are used where appropriate and available. This affects the names of the days and months.

The as.Date methods accept character strings, factors, logical NA and objects of classes "POSIXlt" and "POSIXct". (The last is converted to days by ignoring the time after midnight in the representation of the time in specified time zone, default UTC.) Also objects of class "date" (from package date) and "dates" (from package chron). Character strings are processed as far as necessary for the format specified: any trailing characters are ignored.

as.Date will accept numeric data (the number of days since an epoch), but only if origin is supplied.

The format and as.character methods ignore any fractional part of the date.

The format and as.character methods return a character vector representing the date. NA dates are returned as NA_character_.

The as.Date methods return an object of class "Date".

Most systems record dates internally as the number of days since some origin, but this is fraught with problems, including

Is the origin day 0 or day 1? As the ‘Examples’ show, Excel manages to use both choices for its two date systems.
If the origin is far enough back, the designers may show their ignorance of calendar systems. For example, Excel's designer thought 1900 was a leap year (claiming to copy the error from earlier DOS spreadsheets), and Matlab's designer chose the non-existent date of ‘January 0, 0000’ (there is no such day), not specifying the calendar. (There is such a year in the ‘Gregorian’ calendar as used in ISO 8601:2004, but that does say that it is only to be used for years before 1582 with the agreement of the parties in information exchange.)

The only safe procedure is to check the other systems values for known dates: reports on the Internet (including R-help) are more often wrong than right.

The default formats follow the rules of the ISO 8601 international standard which expresses a day as "2001-02-03".

If the date string does not specify the date completely, the returned answer may be system-specific. The most common behaviour is to assume that a missing year, month or day is the current one. If it specifies a date incorrectly, reliable implementations will give an error and the date is reported as NA. Unfortunately some common implementations (such as glibc) are unreliable and guess at the intended meaning.

Years before 1CE (aka 1AD) will probably not be handled correctly.

International Organization for Standardization (2004, 1988, 1997, ...) ISO 8601. Data elements and interchange formats – Information interchange – Representation of dates and times. For links to versions available on-line see (at the time of writing) http://www.qsl.net/g1smd/isopdf.htm.

Date for details of the date class; locales to query or set a locale.

Your system's help pages on strftime and strptime to see how to specify their formats. Windows users will find no help page for strptime: code based on glibc is used (with corrections), so all the format specifiers described here are supported, but with no alternative number representation nor era available in any locale.

## locale-specific version of the date
format(Sys.Date(), "%a %b %d")

## read in date info in format 'ddmmmyyyy'
## This will give NA(s) in some locales; setting the C locale
## as in the commented lines will overcome this on most systems.
## lct <- Sys.getlocale("LC_TIME"); Sys.setlocale("LC_TIME", "C")
x <- c("1jan1960", "2jan1960", "31mar1960", "30jul1960")
z <- as.Date(x, "%d%b%Y")
## Sys.setlocale("LC_TIME", lct)
z

## read in date/time info in format 'm/d/y'
dates <- c("02/27/92", "02/27/92", "01/14/92", "02/28/92", "02/01/92")
as.Date(dates, "%m/%d/%y")

## date given as number of days since 1900-01-01 (a date in 1989)
as.Date(32768, origin = "1900-01-01")
## Excel is said to use 1900-01-01 as day 1 (Windows default) or
## 1904-01-01 as day 0 (Mac default), but this is complicated by Excel
## incorrectly treating 1900 as a leap year.
## So for dates (post-1901) from Windows Excel
as.Date(35981, origin = "1899-12-30") # 1998-07-05
## and Mac Excel
as.Date(34519, origin = "1904-01-01") # 1998-07-05
## (these values come from http://support.microsoft.com/kb/214330)

## Experiment shows that Matlab's origin is 719529 days before ours,
## (it takes the non-existent 0000-01-01 as day 1)
## so Matlab day 734373 can be imported as
as.Date(734373, origin = "1970-01-01") - 719529 # 2010-08-23
## (value from 
## http://www.mathworks.de/de/help/matlab/matlab_prog/represent-date-and-times-in-MATLAB.html)

## Time zone effect
z <- ISOdate(2010, 04, 13, c(0,12)) # midnight and midday UTC
as.Date(z) # in UTC
## these time zone names are common
as.Date(z, tz = "NZ")
as.Date(z, tz = "HST") # Hawaii