| 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 similar underlying code, used by Linux, OS X, Solaris, AIX, FreeBSD, Sun Java >= 1.4 and Tcl >= 8.5, and installed with R on Windows. Unfortunately there are many system-specific errors in the implementation.
It should be possible to set the time zone via the environment variable TZ: see the section on ‘Time zone names’ for suitable values.
It is not in general possible to retrieve the system's own name(s) for
the current time zone if environment variable TZ is not set, but
on Windows 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). (Some hints for retrieving the name from a
Unix-alike OS are in the examples.)
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 rule changes late or not at all.
The most 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 but for some places there had been time-zone changes before 1902.
64-bit representations are becoming common but are still pretty
unreliable.
Sys.timezone returns an OS-specific character string, possibly
NA or an empty string (which on some OSes means UTC)..
For Windows 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 are other more
user-friendly ways to specify time zones.
Many systems 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
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 time-zone
history since 1970 have specific names, but those with different
earlier histories may not. Each time zone has one or two (the second
for DST) abbreviations used when formatting times.
Note that that the abbreviations have changed over the years: for example France used PMT (‘Paris Mean Time’) from 1891 to 1911 then CET/CEST, butWET/WEST for parts of 1940–5. Software will generally use the current abbreviation(s), and the POSIX standard allows only one or two abbreviations per time zone.
Most Unix-alikes use the Olson database. The system-specific
location in the file system varies, e.g. ‘/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). See also
http://en.wikipedia.org/wiki/Zone.tab.
Many systems support time zones 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.
Since 2007 there has been considerable disruption over changes to the timings of the DST transitions, aimed at energy conservation. These often have short notice and time-zone databases may not be up to date.
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.
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.
Sys.timezone()
## need to find a suitable file path (if any) for your system
tzdirs <- c("/usr/share/zoneinfo", # Linux, OS X, FreeBSD
"/usr/lib/zoneinfo", # early glibc
"/usr/share/lib/zoneinfo", # AIX?
"/usr/local/etc/zoneinfo", # tzcode default
"/etc/zoneinfo", "/usr/etc/zoneinfo")
tzfiles <- c(file.path(tzdirs, "zone.tab"),
"/usr/share/lib/zoneinfo/tab/zone_sun.tab") # Solaris
if (any(have <- file.exists(tzfiles))) {
tzfile <- tzfiles[have][1]
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)
}
## Try to find the system's internal name for the current time zone.
if(nzchar(lt <- Sys.getenv("TZ"))) { # always set on Solaris, AIX
print(lt)
} else {
lt <- normalizePath("/etc/localtime") # Linux, OS X, ...
if (grepl(pat <- "^/usr/share/zoneinfo/", lt))
print(sub(pat, "", lt))
}