Cross-timezone date calculations using the “date”command

Working remotely for a timezone-distributed company poses an interesting challenge: that of having to figure out dates and times for people in different timezones. This involves not only the relatively trivial “what time is it now in A_FARAWAY_PLACE”, but “what time, in FARAWAY_PLACE_X, will it be in FARAWAY_PLACE_Z” and other fun things.

There are a handful of websites that have handy tools to do these conversions for you; but a problem I’ve found is that the web is going to the crapper, and these sites often have confusing UIs concocted by some javascript-crazed, CSS-infected webmonkey; and often they are completely swamped and rendered unusable by a rising tide of ads and other aggressive content (oh and some won’t let you do anything until you agree to them storing information in cookies in your browser – which they then bafflingly don’t use to store the *PREFERENCE* you have selected , so like a forgetful vampire, they ask you every single time if you want to accept their silly cookies).

I’ve known how to use the “date” command to show the date on a different place/timezone, which is already a huge timesaver:

but – today I was trying to answer “what time in TZ=”America/Chicago” is 1 PM, on Tuesday, in “UK/London“. This is interesting because it’s conversion between two timezones which are not the one I’m in, of a date/time in the future. So I was checking date’s man page for “how to convert a specific point in time”, when I realized date can do this for you! Right in the man page there’s this example:

Show the local time for 9AM next Friday on the west coast of the US

so then I combined that with the earlier one to come up with:

This combines

  • TZ argument to calculate dates for a specific timezone, not the current one
  • –date parameter to “display time described by STRING, not ‘now'”
  • Descriptive time specifications (1:00 PM next Tuesday – this is a pseudo-human-readable format which is not entirely intuitive – info date has the specifics)
  • TZ support *inside* the descriptive specification

And a list of known timezones can be obtained with timedatectl list-timezones.

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