Hi Marcelo,
To help you tie in how the formulas that were supplied as answers
work, you should be aware that time is a fraction of a day. So 6
hours is .25 days, and 20 minutes is =20/(24*60) of a day.

Dates are measured as days past a reference date. You can
read more about Date and Time and their calculations in
http://www.mvps.org/dmcritchie/excel/datetime.htm
http://www.cpearson.com/excel/datetime.htm

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HTH,
David McRitchie, Microsoft MVP - Excel [site changed Nov. 2001]
My Excel Pages: http://www.mvps.org/dmcritchie/excel/excel.htm
Search Page: http://www.mvps.org/dmcritchie/excel/search.htm