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How Excel Stores Dates and Times

Summary: Excel stores dates and times internally using what is called a serial number. This tip explains how that serial number is derived. (This tip works with Microsoft Excel 97, Excel 2000, Excel 2002, Excel 2003, and Excel 2007.)

Internally, Excel stores a date or time as a number. The whole part of the number (the part to the left of the decimal point) represents the number of days since an arbitrary starting point (typically January 1, 1900). The decimal portion (the part to the right of the decimal point) represents the time for that date. These internal representations of dates and times are often referred to as serial numbers.

To see how this works, enter the number 23 in a cell. If you have not previously formatted the cell, Excel uses the General format, displaying the number simply as 23. If you later format this cell using a date format—m/d/yy, for instance—Excel changes the display to 1/23/00, or January 23, 1900.

The portion to the right of the decimal point represents a fractional portion of a day. Thus, a single second would be equal to approximately 0.00001157407, since that is equal to 1 (a day) divided by 86,400 (the number of seconds in a day).

Since Excel stores dates and times as serial numbers, you can do math on them. For instance, if you wanted to determine the number of days between two dates, or the amount of time between two times, simply subtract them from each other. The result is the number of days and fractions of days between the two.

ExcelTips is your source for cost-effective Microsoft Excel training. This tip (2176) applies to Microsoft Excel versions: 97 | 2000 | 2002 | 2003 | 2007

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