I hope someone can point me to the right direction,

I have rows upon rows of records on who shipped what and at what time. I
want to see the elapse time for each shipment by user. The problem is that I
can do a sort of the time and subtract the time in row 1 from row 2 to get
the time, but I want to narrow it down by user so this approach would not be
a good idea. So if row 1 had user x shipping and row 2 had user y shipping
this method would not work.

I am wondering if there is a macro or formula to subtract the elapse time
for each shipment by user, for example:

Time User Elapse Time by user
9:32:00 AM Bob 0:04:00
9:31:00 AM Joe 0:11:11
9:28:00 AM Bob 0:09:00
9:20:00 AM Joe 0:02:00
9:19:00 AM Bob ???
9:18:00 AM Joe ???

(the reason why I have the last 2 records as ? is because I do not have any
more data before this shipment to compare it to)
The time is sorted in descending order, if threre is a way of coming up with
the elapse w/o sorting that would be awesome.

Thanks for any help!!!