Date-time Conversion Functions
Functions to manipulate objects of classes
"POSIXct" representing calendar dates and times.
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An object to be converted.
A time zone specification to be used for the conversion,
if one is required. System-specific (see time zones),
further arguments to be passed to or from other methods.
character string giving a date-time format as used
a date-time object, or something which can be coerced by
as.POSIX* functions convert an object to one of the two
classes used to represent date/times (calendar dates plus time to the
nearest second). They can convert a wide variety of objects,
including objects of the other class and of classes
"date" (from package date),
"dates" (from package
chron) to these classes. Dates without times are
treated as being at midnight UTC.
They can also convert character strings of the formats
"2001/02/03" optionally followed by
white space and a time in the format
"14:52:03". (Formats such as
"01/02/03" are ambiguous
but can be converted via a format specification by
strptime.) Fractional seconds are allowed.
format can be specified for character vectors or
factors: if it is not specified and no standard format works for
NA inputs an error is thrown.
format is specified, remember that some of the format
specifications are locale-specific, and you may need to set the
LC_TIME category appropriately via
Sys.setlocale. This most often affects the use of
%B (month names) and
NAs can be converted to either of the classes, but no
other logical vectors can be.
If you are given a numeric time as the number of seconds since an epoch, see the examples.
Character input is first converted to class
strptime: numeric input is first converted to
"POSIXct". Any conversion that needs to go between the two
date-time classes requires a time zone: conversion from
"POSIXct" will validate times in the
selected time zone. One issue is what happens at transitions
to and from DST, for example in the UK
NA, but the second could be interpreted as either BST or GMT (and common OSes give both possible values). Note too (see
strftime) that OS facilities may not format invalid times correctly.
as.POSIXlt return an object of the
appropriate class. If
tz was specified,
will give an appropriate
"tzone" attribute. Date-times known
to be invalid will be returned as
Some of the concepts used have to be extended backwards in time (the
usage is said to be ‘proleptic’). For example, the origin of
time for the
"POSIXct" class, ‘1970-01-01 00:00.00 UTC’,
is before UTC was defined. More importantly, conversion is done
assuming the Gregorian calendar which was introduced in 1582 and not
used universally until the 20th century. One of the
re-interpretations assumed by ISO 8601:2004 is that there was a year
zero, even though current year numbering (and zero) is a much later
concept (525 AD for year numbers from 1 AD).
If you want to extract specific aspects of a time (such as the day of
the week) just convert it to class
"POSIXlt" and extract the
relevant component(s) of the list, or if you want a character
representation (such as a named day of the week) use the
If a time zone is needed and that specified is invalid on your system, what happens is system-specific but attempts to set it will probably be ignored.
Conversion from character needs to find a suitable format unless one is supplied (by trying common formats in turn): this can be slow for long inputs.
DateTimeClasses for details of the classes;
strptime for conversion to and from character
Sys.timezone for details of the (system-specific) naming
of time zones.
locales for locale-specific aspects.
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(z <- Sys.time()) # the current datetime, as class "POSIXct" unclass(z) # a large integer floor(unclass(z)/86400) # the number of days since 1970-01-01 (UTC) (now <- as.POSIXlt(Sys.time())) # the current datetime, as class "POSIXlt" unlist(unclass(now)) # a list shown as a named vector now$year + 1900 # see ?DateTimeClasses months(now); weekdays(now) # see ?months ## suppose we have a time in seconds since 1960-01-01 00:00:00 GMT ## (the origin used by SAS) z <- 1472562988 # ways to convert this as.POSIXct(z, origin = "1960-01-01") # local as.POSIXct(z, origin = "1960-01-01", tz = "GMT") # in UTC ## SPSS dates (R-help 2006-02-16) z <- c(10485849600, 10477641600, 10561104000, 10562745600) as.Date(as.POSIXct(z, origin = "1582-10-14", tz = "GMT")) ## Stata date-times: milliseconds since 1960-01-01 00:00:00 GMT ## format %tc excludes leap-seconds, assumed here ## For format %tC including leap seconds, see foreign::read.dta() z <- 1579598122120 op <- options(digits.secs = 3) # avoid rounding down: milliseconds are not exactly representable as.POSIXct((z+0.1)/1000, origin = "1960-01-01") options(op) ## Matlab 'serial day number' (days and fractional days) z <- 7.343736909722223e5 # 2010-08-23 16:35:00 as.POSIXct((z - 719529)*86400, origin = "1970-01-01", tz = "UTC") as.POSIXlt(Sys.time(), "GMT") # the current time in UTC ## These may not be correct names on your system as.POSIXlt(Sys.time(), "America/New_York") # in New York as.POSIXlt(Sys.time(), "EST5EDT") # alternative. as.POSIXlt(Sys.time(), "EST" ) # somewhere in Eastern Canada as.POSIXlt(Sys.time(), "HST") # in Hawaii as.POSIXlt(Sys.time(), "Australia/Darwin")
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