"Harlan Grove" wrote...
....
If the ranges above were A1:A7 for start date/time and B1:B7 for end
date/time with headings in A1:B1, then try this formula in C3 (yes, col C
row *3*).

=MAX(B$1:B2)-MAX(IF(COUNT(B$1:B1),B$1:B1,0),A2)

Then copy C3 and paste into C2:C7.

Change the C3 formula to

=MAX(B$2:B3)-MAX(IF(COUNT(B$2:B2),B$2:B2,0),A3)