A Date Bug

Excel incorrectly assumes that the year 1900 was a leap year. This means Excel's internal date system believes there was a 29 Feb 1900, when there wasn't! The most surprising part is that Microsoft did this intentionally, or so they say! More information is available at http://support.microsoft.com/default.aspx?scid=kb;EN-US;q181370.

Here are some additional good links to information on dates and times:

HOW TO: Use Dates and Times in Excel 2000
http://support.microsoft.com/default.aspx?scid=kb;en-us;Q214094#6

Text or Number Converted to Unintended Number Format
http://support.microsoft.com/default.aspx?scid=kb;en-us;Q214233

Maximum Times in Microsoft Excel
http://support.microsoft.com/default.aspx?scid=kb;en-us;Q214386

Dates and Times Displayed as Serial Numbers When Viewing Formulas
http://support.microsoft.com/default.aspx?scid=kb;en-us;Q241072

Controlling and Understanding Settings in the Format Cells Dialog Box
http://support.microsoft.com/default.aspx?scid=kb;en-us;Q264372

How to Use Dates and Times in Microsoft Excel
http://support.microsoft.com/default.aspx?scid=kb;en-us;214094

Dates and times are probably one of the most confusing areas within Excel. Armed with this information, hopefully you will understand more about their many quirks and have an easier time dealing with them.