Putting my timesheets in order today, I finally figured out how to make Excel deal correctly with time durations. The default is to treat values as a date/time, formatted as hh:mm. So a value such as 37:00 – meant as a duration – is displayed as 13:00 (1 PM the following day). To correct this, choose custom cell formatting, and enter the format as [h]:mm.

In OpenOffice Calc, [H]:MM is the default format for a time value (tested with 3.0), so durations work out of the box. For something pre-cooked, the OpenOffice Documentation site has a timesheet template by Vivian Lal.