R: Time Zones

timezones {base} | R Documentation

## Time Zones

### Description

Information about time zones in R. `Sys.timezone` returns the current time zone.

### Usage

    Sys.timezone()

### Details

Time zones are a system-specific topic, but these days almost all R platforms use the same underlying code, used by Linux, Mac OS X, Solaris, AIX, FreeBSD, Sun Java >= 1.4 and Tcl >= 8.5, and installed with R on Windows.

It is not in general possible to retrieve the system's own name(s) for the current timezone, but `Sys.timezone` will retrieve the name it uses for the current time (and the name may differ depending on whether daylight saving time is in effect).

On most platforms it is possible to set the time zone via the environment variable TZ: see the section on ‘Time zone names’ for suitable values.

Note that the principal difficulty with time zones is their individual history: over the last 100 years places have changed their affiliation between major time zones, have opted out of (or in to) DST in various years or adopted rule changes late or not at all. This often involves tiny administrative units in the US/Canada: Iowa had 23 different implementations of DST in the 1960's!

Time zones did not come into use until the second half of the nineteenth century, and DST was first introduced in the early twentieth century, most widely during the First World War (in 1916). The most common implementation of `POSIXct` is as signed 32-bit integers and so only goes back to the end of 1901: on such systems R assumes that dates prior to that are in the same time zone as they were in 1902.

### Value

`Sys.timezone` returns an OS-specific character string, possibly an empty string. Typically this is an abbreviation such as `"EST"`.

### Time zone names

Where OSes describe their valid time zones can be obscure. The help for the C function `tzset` can be helpful, but it can also be inaccurate. There is a cumbersome POSIX specification (listed under environment variable TZ at http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08), which is often at least partially supported, but there usually are other more user-friendly ways to specify timezones.

Many systems make use of a timezone database compiled by Arthur Olson, in which the preferred way to refer to a time zone by a location (typically of a city) e.g. `Europe/London`, `America/Los_Angeles`, `Pacific/Easter`. Some traditional designations are also allowed such as `EST5EDT` or `GB`. (Beware that some of these designations may not be what you think: in particular `EST` is a time zone used in Canada without daylight savings time, and not `EST5EDT` nor (Australian) Eastern Standard Time.) The designation can also be an optional colon prepended to the path to a file giving complied zone information (and the examples above are all files in a system-specific location). See http://www.twinsun.com/tz/tz-link.htm for more details and references. By convention, regions with a unique timezone history since 1970 have specific names, but those with different earlier histories may not.

For most Unix-alikes use the Olson database. The system-specific default location in the file system varies, e.g. ‘/usr/share/zoneinfo’ (Linux, Mac OS X, FreeBSD), ‘/usr/share/lib/zoneinfo’ (Solaris, AIX), ‘/usr/etc/zoneinfo’, .... It is likely that there is a file ‘zone.tab’ in that directory listing the locations known as time-zone names (but not for example `EST5EDT`). See also http://en.wikipedia.org/wiki/Zone.tab.

Many systems support timezones of the form GMT+n and GMT-n, which are at a fixed offset from UTC (hence no DST). Contrary to some usage (but consistent with names such as PST8PDT), negative offsets are times ahead of (east of) UTC, positive offsets are times behind (west of) UTC.

### Note

There is currently (since 2007) considerable disruption over changes to the timings of the DST transitions, aimed at energy conservation. These often have short notice and timezone databases may not be up to date (even if the OS has been updated recently).

Note that except on Windows, the operation of time zones is an OS service, and even on Windows a third-party database is used and can be updated (see the section on ‘Time zone names’). Incorrect results will never be an R issue, so please ensure that you have the courtesy not to blame R for them.

### See Also

`Sys.time`, `as.POSIXlt`.

http://en.wikipedia.org/wiki/Time_zone and http://www.twinsun.com/tz/tz-link.htm for extensive sets of links.

### Examples

    Sys.timezone()

    ## Not run:
    ## need to supply a suitable file path (if any) for your system
    tzfile <- "/usr/share/zoneinfo/zone.tab"
    tzones <- read.delim(tzfile, row.names = NULL, header = FALSE,
        col.names = c("country", "coords", "name", "comments"),
        as.is = TRUE, fill = TRUE, comment.char = "#")
    str(tzones$name)

    ## End(Not run)

[Package base version 2.15.1 Index]