5.In the Format Cells box, click Custom in the Category list. 6.In the Type box, at the top of the list of formats, type [h]:mm;@ and then click OK. (That’s a colon after [h], and a semicolon after mm.) YOu can then add hours. The format will be in the Type list the next time you need it.
From MS, works well.
http://office.microsoft.com/en-us/excel-help/add-or-subtract-time-HA102809662.aspx