Normally, dates are interpreted using the rules of the current time
zone, which in turn are specified by the TZ environment
variable, or by a system default if TZ is not set. To specify a
different set of default time zone rules that apply just to one date,
start the date with a string of the form ‘TZ="rule"’. The
two quote characters (‘"’) must be present in the date, and any
quotes or backslashes within rule must be escaped by a
backslash.
For example, with the GNU date command you can
answer the question “What time is it in New York when a Paris clock
shows 6:30am on October 31, 2019?” by using a date beginning with
‘TZ="Europe/Paris"’ as shown in the following shell transcript:
$ export TZ="America/New_York" $ date --date='TZ="Europe/Paris" 2019-10-31 06:30' Sun Oct 31 01:30:00 EDT 2019
In this example, the --date operand begins with its own
TZ setting, so the rest of that operand is processed according
to ‘Europe/Paris’ rules, treating the string ‘2019-10-31
06:30’ as if it were in Paris. However, since the output of the
date command is processed according to the overall time zone
rules, it uses New York time. (Paris was normally six hours ahead of
New York in 2019, but this example refers to a brief Halloween period
when the gap was five hours.)
A TZ value is a rule that typically names a location in the
‘tz’ database.
A recent catalog of location names appears in the
TWiki Date and Time
Gateway. A few non-GNU hosts require a colon before a
location name in a TZ setting, e.g.,
‘TZ=":America/New_York"’.
The ‘tz’ database includes a wide variety of locations ranging
from ‘Arctic/Longyearbyen’ to ‘Antarctica/South_Pole’, but
if you are at sea and have your own private time zone, or if you are
using a non-GNU host that does not support the ‘tz’
database, you may need to use a POSIX rule instead. Simple
POSIX rules like ‘UTC0’ specify a time zone without
daylight saving time; other rules can specify simple daylight saving
regimes. See Specifying the Time Zone with TZ in The GNU C Library.