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Office 2007 Bible by Lisa A. Bucki, Gavin Powell, Michael R. Irwin, Peter G. Aitken, Michael R. Groh, Cary N. Prague, Faithe Wempen, Herb Tyson, John Walkenbach

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How Excel Handles Dates and Times

This section presents a quick overview of how Excel deals with dates and times. It includes coverage of the Excel program’s date and time serial number system, and it offers tips for entering and formatting dates and times.

Understanding date serial numbers

To Excel, a date is simply a number. More precisely, a date is a serial number that represents the number of days since the fictitious date of January 0, 1900. A serial number of 1 corresponds to January 1, 1900; a serial number of 2 corresponds to January 2, 1900, and so on. This system makes it possible to deal with dates in formulas. For example, you can create a formula to calculate the number of days between two dates (just subtract one from the other).

Excel supports dates from January 1, 1900, through December 31, 9999 (serial number = 2,958,465).

You may wonder about January 0, 1900. This nondate (which corresponds to date serial number 0) is actually used to represent times that aren’t associated with a particular day. This nondate business becomes clear later in this chapter (see the section “Entering times”).

Choose Your Date System: 1900 or 1904

Excel actually supports two date systems: the 1900 date system and the 1904 date system. Which system you use in a workbook determines what date serves as the basis for dates. The 1900 date system uses January 1, 1900, as the day assigned to date serial number 1. The 1904 date system uses January 1, 1904, as the base date. By default, Excel ...

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