How can I have `date` output the time from a different timezone?

  • 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 ?

    EDT is not a recognized timezone- in Linux, at least. See my "Be careful!" answer below for the whole ugly story. If EDT is recognized on your brand of *NIX, you should still exercise caution and double-check your timezone string.

    See also this answer which searches /usr/share/zoneinfo and shows the time in zones matching your query.

  • JRFerguson

    JRFerguson Correct answer

    8 years ago

    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.

    not sure if am I wrong, but `export` looks better than `env` command

    @JRFerguson - If a timezone is not included in the list e.g. Las Vegas or Nevada, what are the alternatives in validating the time?

    @Motivated If your city or state isn't included, choose one that uses the same zone info as yours. For instance, I'm in Florida, which uses the same timezone as New York, so my machines synchronize to New York time.

    @JRFerguson - That assumes that the person is aware of the various timezones and is able to use an alternative e.g. New York in your case. Would that be the only option? For example, i don't live in Europe and i won't know the overlapping timezones.

    @Motivated You can use something like this site to assist you in finding other points in your zone.

    @JRFerguson Maybe you can. But then that shouldn't be necessary. What if there is no Internet connectivity? What if the person is unaware of such a website? What if ... It shouldn't be necessary and at best what you propose is a workaround. That's not the right way to go about it. I'm not saying that you're behind this I'm just saying that it's not the right way (I hope that makes sense - I'm dead tired).

License under CC-BY-SA with attribution


Content dated before 6/26/2020 9:53 AM