Errata.... I wrote:
Assuming -12.00 represents hours, it is in J3, and the
data starts in row 4, the formula in J4 might be: =J3 +
IF(H4<"",H4*24,IF(I4<"",-I4*24,0))

Just notice that you have a "break" -- a blank row between weeks. To make
it easy to copy the formula down, do the following:

=IF(G4="","",IF(J3="",J2,J3)+IF(H4<"",H4*24,IF(I4 <"",-I4*24,0)))