Sys.timezone returns
the name of the current time zone.
Sys.timezone(location = TRUE)
OlsonNames()"UTC" and its synonym "GMT" are accepted on all
platforms. 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 are other more
user-friendly ways to specify time zones. Almost all R platforms make use of a time-zone database originally
compiled by Arthur David Olson and now managed by IANA, in which the
preferred way to refer to a time zone is 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 expect: in particular EST is a
time zone used in Canada without daylight saving 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 time-zone history
since 1970 have specific names in the database, but those with
different earlier histories may not. Each time zone has one or two
(the second for DST) abbreviations used when formatting times. The abbreviations used have changed over the years: for example France
used PMT (‘Paris Mean Time’) from 1891 to 1911 then
WET/WEST up to 1940 and CET/CEST from 1946. (In almost
all time zones the abbreviations have been stable since 1970.) The
POSIX standard allows only one or two abbreviations per time zone, so
you may see the current abbreviation(s) used for older times. The function OlsonNames returns the time-zone names known to
the Olson/IANA database on the current system. The system-specific
location in the file system varies, e.g.\ifelse{latex}{\out{~}}{ } ‘/usr/share/zoneinfo’
(Linux, OS X, FreeBSD), ‘/usr/share/lib/zoneinfo’ (Solaris, AIX),
.... It is likely that there is a file named something like
‘zone.tab’ under that directory listing the locations known as
time-zone names (but not for example EST5EDT): this is read by
OlsonNames. See also
https://en.wikipedia.org/wiki/Zone.tab. Where R was configured with option --with-internal-tzcode
(the default on OS X and Windows: recommended on Solaris), the database at
file.path(R.home("share"), "zoneinfo") is used by default: file
‘VERSION’ in that directory states the version. Environment
variable TZDIR can be used to point to a different
‘zoneinfo’ directory: this is also supported by the native
services on some OSes, e.g.\ifelse{latex}{\out{~}}{ } Linux).
windows An attempt is made (once only per session) to map Windows' idea of the
current time zone to a location, following a version of
http://unicode.org/repos/cldr/trunk/common/supplemental/windowsZones.xml
with additional values deduced from the Windows Registry and documentation.
It can be overridden by setting the TZ environment variable
before any date-times are used in the session. Most platforms support time zones of the form GMT+n and
GMT-n, which assume at a fixed offset from UTC (hence no DST).
Contrary to some expectations (but consistent with names such as
PST8PDT), negative offsets are times ahead of (east of) UTC,
positive offsets are times behind (west of) UTC. Immediately prior to the advent of legislated time zones, most people
used time based on their longitude (or that of a nearby town), known
as ‘Local Mean Time’ and abbreviated as LMT in the
databases: in many countries that was codified with a specific name
before the switch to a standard time. For example, Paris codified its
LMT as ‘Paris Mean Time’ in 1891 (to be used throughout
mainland France) and switched to GMT+0 in 1911. Some systems (notably Linux) have tzselect command which
allows the interactive selection of a supported time zone name. It should be possible to set the time zone via the environment
variable TZ: see the section on ‘Time zone names’ for
suitable values. Sys.timezone() will return the value of
TZ if set (and on some OSes it is always set), otherwise it will
try to retrieve a value which if set for TZ would give the
current time zone. This is not in general possible, and
Sys.timezone(FALSE) on Windows will retrieve the abbreviation
used for the current time.
If TZ is set but empty or invalid, most platforms default to UTC, the time zone colloquially known as GMT (see https://en.wikipedia.org/wiki/Coordinated_Universal_Time). (Some but not all platforms will give a warning for invalid time zones.)
Time zones did not come into use until the second half of the
nineteenth century and were not widely adopted until the twentieth,
and daylight saving time (DST, also known as summer
time) was first introduced in the early twentieth century, most
widely in 1916. 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 DST rule changes late or not at all.
A quite common system 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. Most of the world had not adopted time zones by
1902 (so used local ‘mean time’ based on longitude) but for a
few places there had been time-zone changes before then. 64-bit
representations are becoming common; unfortunately on some 64-bit OSes
(notably OS X) the database information is 32-bit and so only
available for the range 1901--2038, and incompletely for the end
years.
Sys.time, as.POSIXlt.https://en.wikipedia.org/wiki/Time_zone and https://www.twinsun.com/tz/tz-link.htm for extensive sets of links.
Sys.timezone()
str(OlsonNames()) ## a few hundred names
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