How can I have `date` output the time from a different timezone?
You can set a timezone for the duration of the query, thusly:
TZ=America/New_York date
Note the whitespace between the TZ setting and the date command. In Bourne-like and rc-like shell, that sets the TZ variable only for the command line. In other shells (csh, tcsh, fish), you can always use the env command instead:
env TZ=America/New_York date
tl;dr
On Linux systems. timezones are defined in files in the /usr/share/zoneinfo directory. This structure is often referred to as the "Olson database" to honor its founding contributor.
The rules for each timezone are defined as text file lines which are then compiled into a binary file. The lines so compiled, define the zone name; a range of data and time during which the zone applies; an offset from UTC for the standard time; and the notation for defining how transition to-and-from daylight saving time occurs, if applicable.
For example, the directory "America" contains the requisite information for New York in the file America/New_York as used, above.
Beware that the specification of a non-existent zone (file name) is silently ignored and UTC times are reported. For example, this reports an incorrect time:
TZ="America/New York" date ### WRONG ###
The Single UNIX Specification, version-3, known as SUSv3 or POSIX-2001, notes that for portability, the character string that identifies the timezone description should begin with a colon character. Thus, we can also write:
TZ=":America/New_York" date
TZ=":America/Los_Angeles" date
As an alternative method to the specification of timezones using a pathname to a description file, SUSv3 describes the POSIX model. In this format, a string is defined as:
std offset [dst[offset][,start-date[/time],end-date[/time]]]
where std is the standard component name and dst is the daylight saving one. Each name consists of three or more characters. The offset is positive for timezones west of the prime meridian and negative for those east of the meridian. The offset is added to the local time to obtain UTC (formerly known as GMT). The start and end time fields indicate when the standard/daylight transitions occur.
For example, in the Eastern United States, standard time is 5-hours earlier than UTC, and we can specify EST5EDT in lieu of America/New_York. These alternatives are not always recognized, however, especially for zones outside of the United States and are best avoided.
HP-UX (an SUSv3 compliant UNIX) uses textual rules in /usr/lib/tztab and the POSIX names like EST5EDT, CST6CDT, MST7MDT, PST8PDT. The file includes all of the historical rules for each time zone, akin to the Olson database.
NOTE: You should be able to find all of the timezones by inspecting the following directory: /usr/share/zoneinfo.
You can do this by manipulating the TZ environment variable. The following will give you the local time for US/Eastern, which will also be smart enough to handle DST when that rolls around:
# all on one line
TZ=":US/Eastern" date +%Y%m%d
The zone name comes from the files and directories inside /usr/share/zoneinfo.
Be careful! Date will happily spit out the time in your CURRENT timezone, if you give it a timezone it doesn't recognize.
Check this out:
-bash-4.2$ env TZ=EDT date
Wed Feb 18 19:34:41 EDT 2015
-bash-4.2$ date
Wed Feb 18 19:34:43 UTC 2015
Note that there is no timezone called EDT. As a matter of fact,
-bash-4.2$ find /usr/share/zoneinfo -name "*EDT*"
returns
/usr/share/zoneinfo/EST5EDT
/usr/share/zoneinfo/posix/EST5EDT
/usr/share/zoneinfo/right/EST5EDT
And this works:
-bash-4.2$ TZ=EST5EDT date
Wed Feb 18 14:36:59 EST 2015
-bash-4.2$ date
Wed Feb 18 19:37:01 UTC 2015
However, if your friend lives in the mystic land of Gobbledygook and its zone info coincides with your own, you can have date output the time in Gobbledygook's zone and it will be happy to do so with nary an exit value to let you know that the zone is not known to it:
-bash-4.2$ TZ=Gobbledygook date
Wed Feb 18 19:37:36 Gobbledygook 2015
-bash-4.2$ echo $?
0