Dreamcatcher - if you want to have the result in hours and minutes (as opposed to hours.decimal hours) there is a really simple trick:
Open a new spreadsheet, type "23:10" in cell A1, "1:00" in A2. Excel, being clever and all, realises that you're typing in "times". Now click in A3, and type:
=SUM(A1:A2)
the answer is "00:10", which is not quite what you'd hoped for.
Right-click cell A3
Select "Format Cells..." from the pop-up menu
On the "Number" tab, select "Custom" at the bottom of the list
Type " [h]:mm " in the "Type" window, then click "OK"
You will find that the answer is now correct.
Ta Daaaaaaaah
This obviates the need for huge numbers of hidden columns, stripping out the minutes and hours, converting to decimal, adding up, then converting back to hh:mm format. Just let Excel do all that hard work.