as.Date: Date Conversion Functions to and from Character

Description

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

Usage

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

Arguments

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.

a time zone name.

…

Further arguments to be passed from or to other methods, including format for as.character and as.Date methods.

Value

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".

Conversion from other Systems

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.

Details

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.

References

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.

Examples

Run this code



## 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

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