I'm having trouble adding up all the sub-totals in my timesheet.

I have a number of sub-totals in h:mm format for each week of the month. I
used the AutoSum function to calculate each of these weekly sub-totals.

When I try to calculate my total hours for the month (i.e. add up all the
weekly sub-totals), I use the forumla: =SUM(C20,C35,C50,C57) . However, Excel
fails to add these up correctly - it simply reproduces the sub-total figure
at C20.

I've checked the fx field and the formula appears correctly. I've also made
sure all the time cells are in the same h:mm format.

Can anyone help me?!