Would like a duration field for events

In addition to being able to specify an end time for events, it would be very helpful to also be able to specify it in terms of an event duration (e.g. 30 mins, 1 hr, etc.).  Google calendar lets you do this, and I find it to be a time-saver and a way to avoid mistakes.  For example, say you create an event starting at 11:00am and ending at 11:30am.  If I need to change this event to be an hour long, I now have to change three fields within the end time: the hour, minute, and the AM/PM field.  May sound minor but I think it would be useful.