This one had me going for a while the other day. I had a cell with a time in it, and just wanted to add 15 minutes to it and place it in the cell next to it. Anyway, the way to do this is to add (or subtract) #Minutes/1440.

For example, if cell A1 has the time 6:15 in it, and you want to add 15 minutes to it for cell A2 simply set it =A1+(15/1440).

The reason for this is that Excel (and Numbers, and.. most likely any other spreadsheet software out there) keeps track of dates, not time – so if you have a date of 6/19/2009, and in the next cell you want 6/20/2009 – you add 1. Well, there are 1440 minutes in a day, so you simply have to add a fraction of a day equal to the amount of time you want to add. So 15/1440 = 0.0104166667 days.