Date-Time Classes

Description

Description of the classes "POSIXlt" and "POSIXct" representing calendar dates and times.

Usage

## S3 method for class 'POSIXct'
print(x, tz = "", usetz = TRUE, ...)

## S3 method for class 'POSIXct'
summary(object, digits = 15, ...)

time + z
z + time
time - z
time1 lop time2

Arguments

Details

There are two basic classes of date/times. Class "POSIXct" represents the (signed) number of seconds since the beginning of 1970 (in the UTC time zone) as a numeric vector. Class "POSIXlt" is a named list of vectors representing

sec

0–61: seconds.

min

0–59: minutes.

hour

0–23: hours.

mday

1–31: day of the month

mon

0–11: months after the first of the year.

year

years since 1900.

wday

0–6 day of the week, starting on Sunday.

yday

0–365: day of the year.

isdst

Daylight Saving Time flag. Positive if in force, zero if not, negative if unknown.

zone

(Optional.) The abbreviation for the time zone in force at that time: "" if unknown (but "" might also be used for UTC).

gmtoff

(Optional.) The offset in seconds from GMT: positive values are East of the meridian. Usually NA if unknown, but 0 could mean unknown.

(The last two components are not present for times in UTC and are platform-dependent: they are supported on platforms based on BSD or glibc (including Linux and macOS) and those using the tzcode implementation shipped with R (including Windows). But they are not necessarily set.). Note that the internal list structure is somewhat hidden, as many methods (including length(x), print() and str) apply to the abstract date-time vector, as for "POSIXct". The classes correspond to the POSIX/C99 constructs of ‘calendar time’ (the time_t data type) and ‘local time’ (or broken-down time, the struct tm data type), from which they also inherit their names. The components of "POSIXlt" are integer vectors, except sec and zone.

"POSIXct" is more convenient for including in data frames, and "POSIXlt" is closer to human-readable forms. A virtual class "POSIXt" exists from which both of the classes inherit: it is used to allow operations such as subtraction to mix the two classes.

Components wday and yday of "POSIXlt" are for information, and are not used in the conversion to calendar time. However, isdst is needed to distinguish times at the end of DST: typically 1am to 2am occurs twice, first in DST and then in standard time. At all other times isdst can be deduced from the first six values, but the behaviour if it is set incorrectly is platform-dependent.

Logical comparisons and some arithmetic operations are available for both classes. One can add or subtract a number of seconds from a date-time object, but not add two date-time objects. Subtraction of two date-time objects is equivalent to using difftime. Be aware that "POSIXlt" objects will be interpreted as being in the current time zone for these operations unless a time zone has been specified.

"POSIXlt" objects will often have an attribute "tzone", a character vector of length 3 giving the time zone name from the TZ environment variable and the names of the base time zone and the alternate (daylight-saving) time zone. Sometimes this may just be of length one, giving the time zone name.

"POSIXct" objects may also have an attribute "tzone", a character vector of length one. If set to a non-empty value, it will determine how the object is converted to class "POSIXlt" and in particular how it is printed. This is usually desirable, but if you want to specify an object in a particular time zone but to be printed in the current time zone you may want to remove the "tzone" attribute (e.g., by c(x)).

Unfortunately, the conversion is complicated by the operation of time zones and leap seconds (according to this version of R's data, 27 days have been 86401 seconds long so far, the last being on (actually, immediately before) 2017-01-01: the times of the extra seconds are in the object .leap.seconds). The details of this are entrusted to the OS services where possible. It seems that some rare systems used to use leap seconds, but all known current platforms ignore them (as required by POSIX). This is detected and corrected for at build time, so "POSIXct" times used by R do not include leap seconds on any platform.

Using c on "POSIXlt" objects converts them to the current time zone, and on "POSIXct" objects drops any "tzone" attributes (even if they are all marked with the same time zone).

A few times have specific issues. First, the leap seconds are ignored, and real times such as "2005-12-31 23:59:60" are (probably) treated as the next second. However, they will never be generated by R, and are unlikely to arise as input. Second, on some OSes there is a problem in the POSIX/C99 standard with "1969-12-31 23:59:59 UTC", which is -1 in calendar time and that value is on those OSes also used as an error code. Thus as.POSIXct("1969-12-31 23:59:59", format = "%Y-%m-%d %H:%M:%S", tz = "UTC") may give NA, and hence as.POSIXct("1969-12-31 23:59:59", tz = "UTC") will give "1969-12-31 23:59:00". Other OSes (including the code used by R on Windows) report errors separately and so are able to handle that time as valid.

The print methods respect options("max.print").

Sub-second Accuracy

Classes "POSIXct" and "POSIXlt" are able to express fractions of a second. (Conversion of fractions between the two forms may not be exact, but will have better than microsecond accuracy.)

Fractional seconds are printed only if options("digits.secs") is set: see strftime.

Valid ranges for times

The "POSIXlt" class can represent a very wide range of times (up to billions of years), but such times can only be interpreted with reference to a time zone.

The concept of time zones was first adopted in the nineteenth century, and the Gregorian calendar was introduced in 1582 but not universally adopted until 1927. OS services almost invariably assume the Gregorian calendar and may assume that the time zone that was first enacted for the location was in force before that date. (The earliest legislated time zone seems to have been London on 1847-12-01.) Some OSes assume the previous use of ‘local time’ based on the longitude of a location within the time zone.

Most operating systems represent POSIXct times as C type long. This means that on 32-bit OSes this covers the period 1902 to 2037. On all known 64-bit platforms and for the code we use on 32-bit Windows, the range of representable times is billions of years: however, not all can convert correctly times before 1902 or after 2037. A few benighted OSes used a unsigned type and so cannot represent times before 1970.

Where possible the platform limits are detected, and outside the limits we use our own C code. This uses the offset from GMT in use either for 1902 (when there was no DST) or that predicted for one of 2030 to 2037 (chosen so that the likely DST transition days are Sundays), and uses the alternate (daylight-saving) time zone only if isdst is positive or (if -1) if DST was predicted to be in operation in the 2030s on that day.

Note that there are places (e.g., Rome) whose offset from UTC varied in the years prior to 1902, and these will be handled correctly only where there is OS support.

There is no reason to suppose that the DST rules will remain the same in the future, and indeed the US legislated in 2005 to change its rules as from 2007, with a possible future reversion. So conversions for times more than a year or two ahead are speculative.

Warnings

Some Unix-like systems (especially Linux ones) do not have environment variable TZ set, yet have internal code that expects it (as does POSIX). We have tried to work around this, but if you get unexpected results try setting TZ. See Sys.timezone for valid settings.

Great care is needed when comparing objects of class "POSIXlt". Not only are components and attributes optional; several components may have values meaning ‘not yet determined’ and the same time represented in different time zones will look quite different.

Currently the order of the list components of "POSIXlt" objects must not be changed, as several C-based conversion methods rely on the order for efficiency.

References

Ripley, B. D. and Hornik, K. (2001) Date-time classes. R News, 1/2, 8–11. https://www.r-project.org/doc/Rnews/Rnews_2001-2.pdf

See Also

Dates for dates without times.

as.POSIXct and as.POSIXlt for conversion between the classes.

strptime for conversion to and from character representations.

Sys.time for clock time as a "POSIXct" object.

difftime for time intervals.

cut.POSIXt, seq.POSIXt, round.POSIXt and trunc.POSIXt for methods for these classes.

weekdays for convenience extraction functions.

Examples

(z <- Sys.time())             # the current date, as class "POSIXct"

Sys.time() - 3600             # an hour ago

as.POSIXlt(Sys.time(), "GMT") # the current time in GMT
format(.leap.seconds)         # the leap seconds in your time zone
print(.leap.seconds, tz = "PST8PDT")  # and in Seattle's

## look at *internal* representation of "POSIXlt" :
leapS <- as.POSIXlt(.leap.seconds)
names(leapS) ; is.list(leapS)
## str() "too smart" -->  need unclass(.):
utils::str(unclass(leapS), vec.len = 7)