Need Macro to figure out time

I have pasted an example of what I have. I need to have a macro be able to subtract the first cell from the last in each set. The sets will constantly change, and I do not know how to tell it to look for the empty cell and use the cell above it to subtract from 1st cell each group - make any sense? Hoping someone can help me out. Thanks.

6/4/2012 18:56:06
6/4/2012 19:31:01
6/4/2012 19:57:28
6/4/2012 20:15:54
6/4/2012 20:53:31
6/4/2012 21:14:21
6/4/2012 21:37:53
6/4/2012 21:53:26
6/4/2012 22:44:11
6/4/2012 22:54:58
6/4/2012 23:32:45
6/5/2012 00:10:24
6/5/2012 00:25:55
6/5/2012 01:06:16
6/5/2012 01:15:28
6/5/2012 01:39:18
6/5/2012 01:58:37
6/5/2012 02:41:43 Need this cell to subtract from 1st cell of this group.

6/5/2012 18:55:58
6/5/2012 19:19:28
6/5/2012 21:22:40
6/5/2012 21:29:36
6/5/2012 22:59:33
6/5/2012 23:04:39
6/6/2012 00:40:38
6/6/2012 00:45:10
6/6/2012 00:49:59
6/6/2012 00:55:55
6/6/2012 01:04:38
6/6/2012 01:09:57
6/6/2012 01:50:54
6/6/2012 01:56:00
6/6/2012 02:05:30
6/6/2012 02:19:17
6/6/2012 02:28:50
6/6/2012 02:40:26
6/6/2012 02:59:53 Need this cell to subtract from 1st cell of this group.

6/6/2012 18:13:04
6/6/2012 18:24:29
6/6/2012 18:34:49
6/6/2012 18:40:05
6/6/2012 18:46:03
6/6/2012 18:47:51
6/6/2012 19:02:13
6/6/2012 19:17:10
6/6/2012 19:19:28
6/6/2012 19:21:26
6/6/2012 19:44:08
6/6/2012 19:58:40
6/6/2012 20:14:04
6/6/2012 20:30:40
6/6/2012 20:58:00
6/6/2012 21:19:04
6/6/2012 21:47:06
6/6/2012 22:05:41
6/6/2012 22:39:04
6/6/2012 23:05:01
6/6/2012 23:29:46
6/6/2012 23:40:19
6/6/2012 23:50:57
6/7/2012 00:19:36
6/7/2012 00:28:48
6/7/2012 00:32:02
6/7/2012 00:37:52
6/7/2012 00:47:54
6/7/2012 00:53:48
6/7/2012 01:06:11
6/7/2012 01:08:52
6/7/2012 01:16:45
6/7/2012 01:31:19
6/7/2012 01:47:14
6/7/2012 02:31:53 Need this cell to subtract from 1st cell of this group.

Awesome... Thank you both very much.