Support a time offset in addition to absolute time

[jmathai]

The --time argument for the update command only supports an absolute time. It would be great to also support an offset (plus or minus) to adjust the date based on its current date.

This could be done by overloading --time and checking for specific syntax or (preferably) adding an --time-offset argument. It would be nice if it supported english string's like PHP's strtotime - there is a python package for it but prefer not to pull it in.