Dates are stored in Excel as serial numbers.
Dates are stored in Excel as serial numbers starting with 1, which represents January 1, 1900. That's when the Excel calendar starts. Each day after that adds one number to the sequence. For example, January 2, 1900 is stored as 2.
That means if you type 8/22/2010, Excel stores the date as 40412 — or 40411 days from January 1, 1900.
Storing dates as serial numbers makes it possible for Excel to do date arithmetic. To find the number of days between two dates, for example, Excel subtracts one serial number from another.
Note You cannot use dates before January 1, 1900 in Excel formulas.