Date Command – How to Output Time from a Different Time Zone

datetimezone

I have a server running with the timezone set to UTC. It seemed like that was generally a good practice (please correct me if I'm wrong).

Anyhow, one of the servers I connect to, in order to scp files, is running on EDT and stores files that I need to copy in the format /path/to/filename/data20120913

I looked at trying to rsync files using something like find's -mtime -1 flag for files modified in the last day, but I didn't have any luck.

I don't mind just using scp to copy the current day's file, but as of right now there is a 4-hour window where running date +%Y%m%d will give a different day on each server and that bugs me a little.

Looking through man date I see that I can have the time output as UTC, but I don't see a way to have it output as another timezone like EDT

I suppose I could also use something like the GNU date extension date -d 20100909 +%s to get the date in seconds from the epoch, apply a manual 4 * 60 * 60 second calculation, and see about rendering that as a date – but then when daylight time kicks in it will still be an hour off.

Is there a simpler way to output the date in a YYYYMMDD format for EDT on a server that is set to UTC ?

Best Answer

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.

Related Question