| timezones {base} | R Documentation |
Information about time zones in R. Sys.timezone returns
the current time zone.
Sys.timezone()
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).
Sys.timezone returns an OS-specific character string, possibly
an empty string. Typically this is an abbreviation such as "EST".
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.
R under Windows uses the Olson database. The current version of the database will be given in file ‘R_HOME\share\zoneinfo\VERSION’. Environment variable TZDIR can be used to point to a later ‘zoneinfo’ directory. A file listing most known time zones can be found at ‘R_HOME\share\zoneinfo\zone.tab’ (see ‘Examples’). By convention, regions with a unique timezone history since 1970 have specific names, but those with different earlier histories may not.
An attempt is made (once only per session) to map Windows' idea of the current time zone to a location, following an earlier version of http://unicode.org/repos/cldr/trunk/common/supplemental/windowsZones.xml If this is not successful, it can be overridden by setting the TZ environment variable.
Windows documents a specification of the form GST-1GDT: this is
interpreted as POSIX-like and hence the ‘US rules’ for changing
to/from DST are applied (and are incorrect for Germany). Versions of
R prior to 2.7.0 used Windows' system functions and hence this form:
it is still accepted for backwards compatibility but was (and remains)
unreliable and gives a warning.
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.
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.
http://en.wikipedia.org/wiki/Time_zone and http://www.twinsun.com/tz/tz-link.htm for extensive sets of links.
Sys.timezone()
tzfile <- file.path(R.home("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)