How to calculate the MTBF for time and date.

I have a worksheet where we want to calculate the MTBF for times and
dates. The user will be manually entering the data (aty this time).

Here is what I have:

Entity Time In Time Out Date MTBF
1 10:00 13:00 11/1/2006
1 15:00 19:00 11/1/2006 2
1 8:00 10:00 11/2/2006 13
11/3/2006
1 0:00 5:00 11/4/2006 38
1 10:00 11:00 11/5/2006
1 9:00 10:00 11/6/2006
17.66666667 Average

I need to find a way to calculate the MTBF and still take into
consiration of the date difference for a 1 month period.

As per the example, I can figure it out manually, though need it to be
more automated. The MTBF is been calculate from the time out and the
time in (difference)


Amny help would be appreciated,

RMc