Functions to convert between character representations and objects of
classes "POSIXlt" and "POSIXct" representing calendar
dates and times.
# S3 method for POSIXct
format(x, format = "", tz = "", usetz = FALSE, …)
# S3 method for POSIXlt
format(x, format = "", usetz = FALSE, …)# S3 method for POSIXt
as.character(x, …)
strftime(x, format = "", tz = "", usetz = FALSE, …)
strptime(x, format, tz = "")
An object to be converted: a character vector for
strptime, an object which can be converted to
"POSIXlt" for strftime.
A character string specifying the time zone to be used for
the conversion. System-specific (see as.POSIXlt), but
"" is the current time zone, and "GMT" is UTC.
Invalid values are most commonly treated as UTC, on some platforms with
a warning.
A character string. The default for the format
methods is
"%Y-%m-%d %H:%M:%S" if any element has a time
component which is not midnight, and "%Y-%m-%d"
otherwise. If options("digits.secs") is set, up to
the specified number of digits will be printed for seconds.
Further arguments to be passed from or to other methods.
logical. Should the time zone abbreviation be appended
to the output? This is used in printing times, and more reliable
than using "%Z".
The format methods and strftime return character vectors
representing the time. NA times are returned as
NA_character_. The elements are restricted to 256 bytes, plus
a time zone abbreviation if usetz is true. (On known platforms
longer strings are truncated at 255 or 256 bytes, but this is not
guaranteed by the C99 standard.)
strptime turns character representations into an object of
class "POSIXlt". The time zone is used to set the
isdst component and to set the "tzone" attribute if
tz != "". If the specified time is invalid (for example
"2010-02-30 08:00") all the components of the result are
NA. (NB: this does means exactly what it says -- if it is an
invalid time, not just a time that does not exist in some time zone.)
Everyone agrees that years from 1000 to 9999 should be printed with 4 digits, but the standards do not define what is to be done outside that range. For years 0 to 999 most OSes pad with zeros or spaces to 4 characters, and Linux outputs just the number.
OS facilities will probably not print years before 1 CE (aka 1 AD)
‘correctly’ (they tend to assume the existence of a year 0: see
https://en.wikipedia.org/wiki/0_(year), and some OSes get them
completely wrong). Common formats are -45 and -045.
Years after 9999 and before -999 are normally printed with five or more characters.
Some platforms support modifiers from POSIX 2008 (and others). On
Linux the format "%04Y" assures a minimum of four characters
and zero-padding. The internal code (as used on Windows and by
default on macOS) uses zero-padding by default, and formats
%_4Y and %_Y can be used for space padding and no
padding.
Offsets from GMT (also known as UTC) are part of the conversion
between timezones and to/from class "POSIXct", but cause
difficulties as they often computed incorrectly.
They conventionally have the opposite sign from time-zone
specifications (see Sys.timezone): positive values are
East of the meridian. Although there have been time zones with
offsets like 00:09:21 (Paris in 1900), and 00:44:30 (Liberia until
1972), offsets are usually treated as whole numbers of minutes, and
are most often seen in RFC 5322 email headers in forms like
-0800 (e.g., used on the Pacific coast of the USA in winter).
Format %z can be used for input or output: it is a character
string, conventionally plus or minus followed by two digits for
hours and two for minutes: the standards say that an empty string
should be output if the offset is unknown, but some systems use the
offsets for the time zone in use for the current year.
Input uses the POSIX function strptime and output the C99
function strftime.
However, not all OSes (notably Windows) provided strptime and
many issues were found for those which did, so since 2000 R has used
a fork of code from glibc. The forked code uses the
system's strftime to find the locale-specific day and month
names and any AM/PM indicator.
On some platforms (including Windows and by default on macOS) the
system's strftime is replaced (along with most of the rest of
the C-level datetime code) by code modified from IANA's tzcode
distribution (https://www.iana.org/time-zones).
The format and as.character methods and strftime
convert objects from the classes "POSIXlt" and
"POSIXct" to character vectors.
strptime converts character vectors to class "POSIXlt":
its input x is first converted by as.character.
Each input string is processed as far as necessary for the format
specified: any trailing characters are ignored.
strftime is a wrapper for format.POSIXlt, and it and
format.POSIXct first convert to class "POSIXlt" by
calling as.POSIXlt (so they also work for class
"Date"). 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 of the longer of these
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 output
formats such as %x and %X, via the setting of
the LC_TIME locale category. The ‘current
locale’ of the descriptions might mean the locale in use at the start
of the R session or when these functions are first used. (For input,
the locale-specific conversions can be changed by calling
Sys.setlocale with category LC_TIME (or
LC_ALL). For output, what happens depends on the OS but
usually works.)
The details of the formats are platform-specific, but the following are
likely to be widely available: most are defined by the POSIX standard.
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
%aAbbreviated weekday name in the current locale on this platform. (Also matches full name on input: in some locales there are no abbreviations of names.)
%AFull weekday name in the current locale. (Also matches abbreviated name on input.)
%bAbbreviated month name in the current locale on this platform. (Also matches full name on input: in some locales there are no abbreviations of names.)
%BFull month name in the current locale. (Also matches abbreviated name on input.)
%cDate and time. Locale-specific on output,
"%a %b %e %H:%M:%S %Y" on input.
%CCentury (00--99): the integer part of the year divided by 100.
%dDay of the month as decimal number (01--31).
%DDate format such as %m/%d/%y: the C99
standard says it should be that exact format (but not all OSes
comply).
%eDay of the month as decimal number (1--31), with a leading space for a single-digit number.
%FEquivalent to %Y-%m-%d (the ISO 8601 date format).
%gThe last two digits of the week-based year
(see %V). (Accepted but ignored on input.)
%GThe week-based year (see %V) as a decimal
number. (Accepted but ignored on input.)
%hEquivalent to %b.
%HHours as decimal number (00--23). As a special exception strings such as 24:00:00 are accepted for input, since ISO 8601 allows these.
%IHours as decimal number (01--12).
%jDay of year as decimal number (001--366).
%mMonth as decimal number (01--12).
%MMinute as decimal number (00--59).
%nNewline on output, arbitrary whitespace on input.
%pAM/PM indicator in the locale. Used in
conjunction with %I and not with %H. An
empty string in some locales (for example on some OSes,
non-English European locales including Russia). The behaviour is
undefined if used for input in such a locale.
Some platforms accept %P for output, which uses a lower-case
version (%p may also use lower case): others will output
P.
%rFor output, the 12-hour clock time (using the
locale's AM or PM): only defined in some locales, and on some OSes
misleading in locales which do not define an AM/PM indicator.
For input, equivalent to %I:%M:%S %p.
%REquivalent to %H:%M.
%SSecond as integer (00--61), allowing for up to two leap-seconds (but POSIX-compliant implementations will ignore leap seconds).
%tTab on output, arbitrary whitespace on input.
%TEquivalent to %H:%M:%S.
%uWeekday as a decimal number (1--7, Monday is 1).
%UWeek of the year as decimal number (00--53) using Sunday as the first day 1 of the week (and typically with the first Sunday of the year as day 1 of week 1). The US convention.
%VWeek of the year as decimal number (01--53) as defined in ISO 8601. 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. (Accepted but ignored on input.)
%wWeekday as decimal number (0--6, Sunday is 0).
%WWeek of the year as decimal number (00--53) using Monday as the first day of week (and typically with the first Monday of the year as day 1 of week 1). The UK convention.
%xDate. Locale-specific on output,
"%y/%m/%d" on input.
%XTime. Locale-specific on output,
"%H:%M:%S" on input.
%yYear without century (00--99). On input, values 00 to 68 are prefixed by 20 and 69 to 99 by 19 -- that is the behaviour specified by the 2004 and 2008 POSIX standards, but they do also say ‘it is expected that in a future version the default century inferred from a 2-digit year will change’.
%YYear with century. Note that whereas there was no zero in the original Gregorian calendar, ISO 8601:2004 defines it to be valid (interpreted as 1BC): see https://en.wikipedia.org/wiki/0_(year). Note that the standards also say that years before 1582 in its calendar should only be used with agreement of the parties involved.
For input, only years 0:9999 are accepted.
%zSigned offset in hours and minutes
from UTC, so -0800 is 8 hours behind UTC. Values up to
+1400 are accepted. (Standard only for output. For input
R currently supports it on all platforms.)
%Z(Output only.) Time zone abbreviation as a character string (empty if not available). This may not be reliable when a time zone has changed abbreviations over the years.
Where leading zeros are shown they will be used on output but are
optional on input. Names are matched case-insensitively on input:
whether they are capitalized on output depends on the platform and the
locale. Note that abbreviated names are platform-specific (although
the standards specify that in the C locale they must be the
first three letters of the capitalized English name: this convention
is widely used in English-language locales but for example the French
month abbreviations are not the same on any two of Linux, macOS, Solaris
and Windows). Knowing what the abbreviations are is essential
if you wish to use %a, %b or %h as part of an
input format: see the examples for how to check.
When %z or %Z is used for output with an
object with an assigned time zone an attempt is made to use the values
for that time zone --- but it is not guaranteed to succeed.
Not in the standards and less widely implemented are
%kThe 24-hour clock time with single digits preceded by a blank.
%lThe 12-hour clock time with single digits preceded by a blank.
%s(Output only.) The number of seconds since the epoch.
%+(Output only.) Similar to %c, often
"%a %b %e %H:%M:%S %Z %Y". May depend on the locale.
For output 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. These are accepted for input, but with
the standard interpretation.
Specific to R is %OSn, which for output gives the seconds
truncated 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 =
0). Further, for strptime %OS will input seconds
including fractional seconds. Note that %S does not read
fractional parts on output.
The behaviour of other conversion specifications (and even if other
character sequences commencing with % are conversion
specifications) is system-specific. Some systems document that the
use of multi-byte characters in format is unsupported: UTF-8
locales are unlikely to cause a problem.
International Organization for Standardization (2004, 2000, …) 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) https://dotat.at/tmp/ISO_8601-2004_E.pdf and http://www.qsl.net/g1smd/isopdf.htm; for information on the current official version, see https://www.iso.org/iso/iso8601.
The POSIX 1003.1 standard, which is in some respects stricter than ISO 8601.
DateTimeClasses for details of the date-time classes; locales to query or set a locale.
Your system's help page on strftime to see how to specify their
formats. (On some systems, including Windows, strftime is
replaced by more comprehensive internal code.)
# NOT RUN {
## 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 non-English 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)
## time zone names 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")
## An RFC 5322 header (Eastern Canada, during DST)
## In a non-English locale the commented lines may be needed.
## prev <- Sys.getlocale("LC_TIME"); Sys.setlocale("LC_TIME", "C")
strptime("Tue, 23 Mar 2010 14:36:38 -0400", "%a, %d %b %Y %H:%M:%S %z")
## Sys.setlocale("LC_TIME", prev)
## Make sure you know what the abbreviated names are for you if you wish
## to use them for input (they are matched case-insensitively):
format(seq.Date(as.Date('1978-01-01'), by = 'day', len = 7), "%a")
format(seq.Date(as.Date('2000-01-01'), by = 'month', len = 12), "%b")
# }
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