From our lead developer:
Actually, in the latest code this is already possible. It will be included in the 2009R1.4 release shortly.
The end date can include hours, minutes seconds like so:
2011-01-01 13:14:23
The start date/time is automatically calculated based on 1) the end date and 2) the time selection (4 hour, 24 hour, etc)
It makes graphing much better to work with for our admins and when we have outages.