Open an Excel document containing a section with time data written in "hh:mm" format, such as "12:30." Click on an open cell off to the side of the first entry in the existing data.

Every date (month, day, year) and time (hour, minute, and second) has an equivalent Serial Number dating back to January 1, 1900 at 00:00:00 (hours, minutes, seconds).

Excel considers dates and times as numbers in cells, making calculation difficult. Excel processes 6 AM as "0.25," because it is one quarter of the way through the day. If you need to add a number ...