Date Arithmetic

The drunken cousin

Working with dates has a few twists.

Excel believes that time began on January 1, 1900.

Each day since then is counted so that September 1, 2003 in Excel-speak would be → 37,865.
9/1/03 7:33 A.M. is a decimal → 37865.31458333333

When you subtract one date from another, for instance 9/1/2003 (A1)minus 7/4/2001 (A2),
Excel displays the odd answer of → 2/27/1902.

Excel formats the result of a formula with the same format as the source cells,
Right-click the formula cell (=A1-A2).
Select Format Cells ..., and then choose a Number format with zero decimals.

The correct number of days → 789 will now be displayed.

Another way is to use the rarely documented DATEDIF function. Chip Pearson calls it "the drunken cousin of the Function family."

=DATEDIF(EarliestDate,LatestDate,Interval)

=DATEDIF(A2,A1,"d")

Here's THE source for date math:
Chip Pearson:
All About Dates
Also:

John Walenbach:
Extended Date Functions Add-In

"Many users are surprised to discover that Excel cannot work with dates prior to the year 1900. The Extended Date Functions add-in (XDate) corrects this deficiency, and allows you to work with dates in the years 0100 through 9999."

MS Knowledge Base:
How To Use Dates and Times in Excel




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