Excel 2007 : Creating an if statement without a fixed range

[Giantsls2]

Hello,
I'm trying to calculate the duration between foreced outages. My goal is create an if statement with multiple criteria to somewhat automate my process.. My data is represented below
Start End Occurrence Duration
1/1/2000 0:00 1/1/2000 3:44 Scheduled 3:44:00
2/3/2000 10:07 2/3/2000 11:56 Scheduled 1:49:00
2/9/2000 18:52 2/9/2000 21:53 Unscheduled 3:01:00
3/6/2000 11:32 3/6/2000 13:24 Scheduled 1:52:00
3/23/2000 12:52 3/23/2000 14:53 Scheduled 2:01:00
4/13/2000 8:30 4/13/2000 16:38 Scheduled 8:08:00
5/19/2000 19:33 5/19/2000 23:06 Unscheduled 3:33:00
6/12/2000 9:21 6/12/2000 9:37 Unscheduled 0:16:00
6/25/2000 20:18 6/26/2000 21:15 Unscheduled 24:57:00
6/30/2000 9:39 6/30/2000 12:43 Scheduled 3:04:00
7/14/2000 18:10 7/14/2000 18:17 Unscheduled 0:07:00
8/15/2000 13:09 8/15/2000 13:20 Unscheduled 0:11:00
9/27/2000 8:37 9/27/2000 15:40 Scheduled 7:03:00
10/19/2000 8:00 10/19/2000 15:03 Scheduled 7:03:00
10/25/2000 21:55 10/26/2000 11:13 Unscheduled 13:18:00

I'm trying to capture the time elapsed between unscheduled events for example the change in time between 2/9/2000 21:53 and 5/19/2000 19:33. I would then like to subtract out the scheduled duration time between these events IE.
Start End Occurrence Duration
3/6/2000 11:32 3/6/2000 13:24 Scheduled 1:52:00
3/23/2000 12:52 3/23/2000 14:53 Scheduled 2:01:00
4/13/2000 8:30 4/13/2000 16:38 Scheduled 8:08:00

Could I write an if statement to capture this criteria but also account for the randomness of unscheduled to scheduled events?