Allow time offsets to be negative, e.g. `at 1530 - 15 minutes`.
Allow time offsets to be negative, e.g. `at 1530 - 15 minutes`.

This is useful if you have been given some time for some event in some
format and you want your computer to do something to prepare for it.
Without having to do time arithmetic in a shellscript.

The syntax matches what the at(1) usually used on Linux supports.

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