On for days and dates, in for duration, at for precise times, including full dates that include the time:
Created in 3 hours and 15 minutes.
Created on the 13th of July
Created on 2013-07-13
Created at 14:35
Created at 2013-07-13 14:35
Created at 2013-07-13T14:35:00Z (when ISO dates are appropriate).
