Need to calculate the difference between two cells in a column. Exemple

A B C
Start 1 date/time
open date/time
: date/time
: date/time
: :
Start2 date/time =date2/time2 - date1/time1
(HH:MM)

I can select two consecutive "Start cells" with "IF"s, but the max of "IF"
supported by Excel is 7. I have no solution if there are more than 7 lines
between tho consecutive "start"s (or any other, like "open" etc..)

I'd be glad to get some help.

68magnolia71 (in france)