The following link to Chip Pearson's site will give you all you will probably
want to know about dates and times in Excel. The text below is a snapshot of
part of the page
http://cpearson.com/excel/datetime.htm
How Excel Stores Dates And Times
Excel stores dates and times as a number representing the number of days since
1900-Jan-0, plus a fractional portion of a 24 hour day: ddddd.tttttt . This is
called a serial date, or serial date-time.
Dates
The integer portion of the number, ddddd, represents the number of days since
1900-Jan-0. For example, the date 29-Jan-2000 is stored as 36,544, since 36,544
days have passed since 1900-Jan-0. The number 1 represents 1900-Jan-1. It
should be noted that the number 0 does not represent 1899-Dec-31. It does not.
If you use the MONTH function with the date 0, it will return January, not
December. Moreover, the YEAR function will return 1900, not 1899.