Date-time Conversion Functions to and from Character

Description

Functions to convert between character representations and objects of classes "POSIXlt" and "POSIXct" representing calendar dates and times.

Usage

## S3 method for class 'POSIXct':
format(x, format = "", tz = "", usetz = FALSE, ...)
## S3 method for class 'POSIXlt':
format(x, format = "", usetz = FALSE, ...)

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

strftime(x, format="", usetz = FALSE, ...)
strptime(x, format, tz = "")

ISOdatetime(year, month, day, hour, min, sec, tz = "")
ISOdate(year, month, day, hour = 12, min = 0, sec = 0, tz = "GMT")

Arguments

Details

The format and as.character methods and strftime convert objects from the classes "POSIXlt" and "POSIXct" (not strftime) to character vectors.

strptime converts character strings to class "POSIXlt": its input x is first coerced to character if necessary. Each string is processed as far as necessary for the format specified: any trailing characters are ignored.

strftime is an alias for format.POSIXlt, and format.POSIXct first converts to class "POSIXlt" by calling as.POSIXlt. Note that only that conversion depends on the time zone.

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 AM/PM indicator (if used) and the separators in formats such as %x and %X.

The details of the formats are system-specific, but the following are defined by the ISO C / POSIX standard for strftime and are likely to be widely available. A conversion specification is introduced by %, usually followed by a single letter or O or E and then a single letter. Any character in the format string not part of a conversion specification is interpreted literally (and %% gives %). Widely implemented conversion specifications include

%a

Abbreviated weekday name in the current locale. (Also matches full name on input.)

%A

Full weekday name in the current locale. (Also matches abbreviated name on input.)

%b

Abbreviated month name in the current locale. (Also matches full name on input.)

%B

Full month name in the current locale. (Also matches abbreviated name on input.)

%c

Date and time, locale-specific.

%d

Day of the month as decimal number (01–31).

%H

Hours as decimal number (00–23).

%I

Hours as decimal number (01–12).

%j

Day of year as decimal number (001–366).

%m

Month as decimal number (01–12).

%M

Minute as decimal number (00–59).

%p

AM/PM indicator in the locale. Used in conjuction with %I and not with %H.

%S

Second as decimal number (00–61), allowing for up to two leap-seconds (but POSIX-compliant OSes will ignore leap seconds).

%U

Week of the year as decimal number (00–53) using the first Sunday as day 1 of week 1.

%w

Weekday as decimal number (0–6, Sunday is 0).

%W

Week of the year as decimal number (00–53) using the first Monday as day 1 of week 1.

%x

Date, locale-specific.

%X

Time, locale-specific.

%y

Year without century (00–99). If you use this on input, which century you get is system-specific. So don't! Often values up to 69 (or 68) are prefixed by 20 and 70(or 69) to 99 by 19.

%Y

Year with century.

%z

(output only.) Offset from Greenwich, so -0800 is 8 hours west of Greenwich.

%Z

(output only.) Time zone as a character string (empty if not available).

Where leading zeros are shown they will be used on output but are optional on input.

Also defined in the current standards but less widely implemented (e.g. not for output on Windows) are

%C

Century (00–99): the integer part of the year divided by 100.

%D

Locale-specific date format such as %m/%d/%y: ISO C99 says it should be that exact format.

%e

Day of the month as decimal number ( 1–31), with a leading space for a single-digit number.

%F

Equivalent to %Y-%m-%d (the ISO 8601 date format).

%g

The last two digits of the week-based year (see %V). (Typically accepted but ignored on input.)

%G

The week-based year (see %V) as a decimal number. (Typically accepted but ignored on input.)

%h

Equivalent to %b.

%k

The 24-hour clock time with single digits preceded by a blank.

%l

The 12-hour clock time with single digits preceded by a blank.

%n

Newline on output, arbitrary whitespace on input.

%r

The 12-hour clock time (using the locale's AM or PM).

%R

Equivalent to %H:%M.

%t

Tab on output, arbitrary whitespace on input.

%T

Equivalent to %H:%M:%S.

%u

Weekday as a decimal number (1–7, Monday is 1).

%V

Week of the year as decimal number (00–53). If the week (starting on Monday) containing 1 January has four or more days in the new year, then it is considered week 1. Otherwise, it is the last week of the previous year, and the next week is week 1. (Typically accepted but ignored on input.)

For output (and possibly input) there are also %O[dHImMUVwWy] which may emit numbers in an alternative locale-dependent format (e.g. roman numerals), and %E[cCyYxX] which can use an alternative ‘era’ (e.g. a different religious calendar). Which of these are supported is OS-dependent.

Specific to R is %OSn, which for output gives the seconds to 0 <= n <= 6 decimal places (and if %OS is not followed by a digit, it uses the setting of getOption("digits.secs"), or if that is unset, n = 3). Further, for strptime %OS will input seconds including fractional seconds.

The behaviour of other conversion specifications (and even if other character sequences commencing with % are conversion specifications) is system-specific.

ISOdatetime and ISOdate are convenience wrappers for strptime, that differ only in their defaults and that ISOdate sets a timezone. (For dates without times it would be better to use the "Date" class.)

Value

The format methods and strftime return character vectors representing the time.
strptime turns character representations into an object of class "POSIXlt". The timezone is used to set the isdst component and (as from R 2.5.0) to set the "tzone" attribute if tz != "".
ISOdatetime and ISOdate return an object of class "POSIXct".

Note

The default formats follow the rules of the ISO 8601 international standard which expresses a day as "2001-02-28" and a time as "14:01:02" using leading zeroes as here. The ISO form uses no space to separate dates and times.

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 unspecified seconds, minutes or hours are zero, and 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.

If the timezone specified is invalid on your system, what happens is system-specific but it will probably be ignored.

OS facilities will probably not print years before 1CE (aka 1AD) correctly.

Remember that in most timezones some times do not occur and some occur twice because of transitions to/from summer time. What happens in those cases is OS-specific.

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; for information on the current official version, see http://www.iso.org/iso/en/prods-services/popstds/datesandtime.html.

See Also

DateTimeClasses for details of the date-time classes; locales to query or set a locale.

Your system's help pages on strftime and strptime to see how to specify their formats. (On some systems strptime is replaced by corrected code from glibc, when all the conversion specifications described here are supported, but with no alternative number representation nor era available in any locale.)

Examples

## locale-specific version of date()
format(Sys.time(), "%a %b %d %X %Y %Z")

## time to sub-second accuracy (if supported by the OS)
format(Sys.time(), "%H:%M:%OS3")

## 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 <- strptime(x, "%d%b%Y")
## Sys.setlocale("LC_TIME", lct)
z

## read in date/time info in format 'm/d/y h:m:s'
dates <- c("02/27/92", "02/27/92", "01/14/92", "02/28/92", "02/01/92")
times <- c("23:03:20", "22:29:56", "01:03:30", "18:21:03", "16:56:26")
x <- paste(dates, times)
strptime(x, "%m/%d/%y %H:%M:%S")

## time with fractional seconds
z <- strptime("20/2/06 11:16:16.683", "%d/%m/%y %H:%M:%OS")
z # prints without fractional seconds
op <- options(digits.secs=3)
z
options(op)

## timezones are not portable, but 'EST5EDT' comes pretty close.
(x <- strptime(c("2006-01-08 10:07:52", "2006-08-07 19:33:02"),
               "%Y-%m-%d %H:%M:%S", tz="EST5EDT"))
attr(x, "tzone")