I have a time-stamp like 7:00:00, which means 7am.

I would like to write a short command that returns 06:45:00, or simply 06:45, preferably using date command so that I can avoid long shell script. Do you have any elegant solution?

I'm also looking for a 24h format. For example, 12:00:00 - 15 minutes = 11:45:00.