Information about time zones in R. Sys.timezone returns
the name of the current time zone.
Sys.timezone(location = TRUE)OlsonNames(tzdir = NULL)
logical: defunct: ignored, with a warning for false values.
The time-zone database to be used: the default is to try known locations until one is found.
Sys.timezone returns an OS-specific character string, possibly
NA or an empty string (which on some OSes means UTC).
This will be a location such as "Europe/London" if one can be
ascertained.
A time zone region may be known by several names: for example "Europe/London" is also known as GB, GB-Eire, Europe/Belfast, Europe/Guernsey, Europe/Isle_of_Man and Europe/Jersey. A few regions are also known by a summary of their time zone, e.g.PST8PDT is an alias for America/Los_Angeles.
OlsonNames returns a character vector, see the examples for
typical cases. It may have an attribute "Version", something
like "2017c".
Names "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 within a ‘time zone region’. 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.
For some time zones abbreviations are like -03 and
+0845: this is done when there is no official abbreviation.
(Negative values are behind (West of) UTC, as for the "%z"
format for strftime.)
The function OlsonNames returns the time-zone names known to
the currently selected Olson/IANA database. The system-specific
location in the file system varies,
e.g./usr/share/zoneinfo (Linux, macOS, 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). See also
https://en.wikipedia.org/wiki/Zone.tab.
Where R was configured with option --with-internal-tzcode
(the default on macOS 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 database: this is also supported by the native
services on some OSes, e.g.Linux using glibc except in
secure modes.
Time zones given by name (via environment variable TZ, in
tz arguments to functions such as as.POSIXlt and
perhaps the system time zone) are loaded from the currently selected
zoneinfo database.
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 Etc/GMT+n and Etc/GMT-n (possibly also without prefix Etc/), which assume 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 a tzselect command which
allows the interactive selection of a supported time zone name. On
systems using systemd (notably Linux), the OS command
timedatectl list-timezones will list all available time zone
names.
There is a system-specific upper limit on the number of bytes in (abbreviated) time-zone names which can be as low as 6 (as required by POSIX). Some OSes allow the setting of time zones with names which exceed their limit, and that can crash the R session.
OlsonNames tries to find an Olson database in known locations.
It might not succeed (when it returns an empty vector with a warning)
and even if it does it might not locate the database used by the
date-time code linked into R. Fortunately names are added rarely
and most databases are pretty complete.
This section is of background interest for users of a Unix-alike, but
may help if an NA value is returned unexpectedly.
Commercial Unixen such as Solaris and AIX set TZ, so the value
when R is started is used.
All other common platforms (Linux, macOS, *BSD) use similar schemes,
either derived from tzcode (currently distributed from
https://www.iana.org/time-zones) or independently coded
(glibc, musl-libc). Such systems read the time-zone
information from a file localtime, usually under /etc
(but possibly under /usr/local/etc or
/usr/local/etc/zoneinfo). As the usual Linux manual page for
localtime says
‘Because the time zone identifier is extracted from the symlink
target name of /etc/localtime, this file may not be a normal
file or hardlink.’
Nevertheless, some Linux distributions (including the one from which
that quote was taken) or sysadmins have chosen to copy a time-zone file
to localtime. For a non-symlink, the ultimate fallback is to
compare that file to all files in the time-zone database.
Some Linux platforms provide two other mechanisms which are tried in
turn before looking at /etc/localtime.
‘Modern’ Linux systems use systemd which
provides mechanisms to set and retrieve the time zone (amongst other
things). There is a command timedatectl to give details.
(Unfortunately RHEL/Centos 6.x are not ‘modern’.)
Debian-derived systems since ca 2007 have supplied a
file /etc/timezone. Its format is undocumented but it
empirically it contains a single line of text naming the time zone.
In each case a sanity check is performed that the time-zone name is the
name of a file in the time-zone database. (The systems probably use
the time-zone file (symlinked to) /etc/localtime, but the
Sys.timezone code
does not check that is the same as the named file in the database. This is deliberate
as they may be from different dates.)
Time zones are a system-specific topic, but these days almost all R platforms use similar underlying code, used by Linux, macOS, Solaris, AIX and FreeBSD, and installed with R on Windows. (Unfortunately there are many system-specific errors in the implementations.) It is possible to use the R sources' version of the code on Unix-alikes as well as on Windows: this is the default for macOS and recommended for Solaris.
It should be possible to set the current 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 initially (and on some OSes it is always set),
otherwise it will try to retrieve from the OS a value which if set for
TZ would give the initial time zone. (‘Initially’ means
before any time-zone functions are used: if TZ is being set to
override the OS setting or if the ‘try’ does not get this
right, it should be set before the R process is started or (probably
early enough) in file .Rprofile).
If TZ is set but 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 values.)
If it is unset or empty the system time zone is used (the one
returned by Sys.timezone).
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. (The UK experimented with DST throughout 1971, only.)
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 macOS) the database information is 32-bit and so only
available for the range 1901--2038, and incompletely for the end
years.
As from R 3.5.0, when a time zone location is first found in a
session, its value is cached in object .sys.timezone in the
base environment.
https://en.wikipedia.org/wiki/Time_zone and http://www.twinsun.com/tz/tz-link.htm for extensive sets of links.
https://data.iana.org/time-zones/theory.html for the ‘rules’ of the Olson/IANA database.
# NOT RUN {
Sys.timezone()
str(OlsonNames()) ## typically close to 600 hundred names,
## typically some acronyms/aliases such as "UTC", "NZ", "MET", "Eire", ..., but
## mostly pairs (and triplets) such as "Pacific/Auckland"
table(sl <- grepl("/", OlsonNames()))
OlsonNames()[ !sl ] # the simple ones
head(Osl <- strsplit(OlsonNames()[sl], "/"))
(tOS1 <- table(vapply(Osl, `[[`, "", 1))) # Continents, countries, ...
table(lengths(Osl))# most are pairs, some triplets
str(Osl[lengths(Osl) >= 3])# "America" South and North ...
# }
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