First selct the cell you want assign a format.
Then from the Menue choose
- Home
- Click on the small arrow besides "Number"
- In the new dialog choose "Number" again, and select the last Item in the list reading "Custom"
It will show you the format which is applied to the selected cell.
- Where it reads "Type" below you can enter the format you desire e.g. [hh]:mm. The field above gives you a preview how your format affects the cell value.
Edit: I have to correct this one: That's nonsense, sorry, formats doesn't affect the value of a cell, only the way it is displayed. The value remains the same!

More about customized formats you will find for example on this page:
http://office.microsoft.com/en-us/ex...010342372.aspx