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Chapter 15. Formatting and Calculating D... > Understanding How Excel Records Date...

Understanding How Excel Records Dates and Times

Excel assigns serial values to days, hours, minutes, and seconds, which makes it possible for you to perform sophisticated date and time arithmetic. The basic unit of time in Excel is the day. Each day is represented by a serial date value. The base date, represented by the serial value 1, is Sunday, January 1, 1900. When you enter a date in your worksheet, Excel records the date as a serial value that represents the number of days between the base date and the specified date. For example, the date January 1, 2004 is represented by the serial value 37,987, representing the number of days between the base date—January 1, 1900—and January 1, 2004.

The time of day is a decimal value that represents the portion of a day that has passed from its beginning—12:00 midnight—to the specified time. Therefore, the time 12:00 noon is represented by the value 0.5, because the difference between midnight and noon is exactly half a day. The time/date combination 2:09:03 PM, October 23, 2004, is represented by the serial value 38283.5896180556, because October 23, 2004 is day number 38,283 (counting January 1, 1900 as day number 1), and the interval between midnight and 2:09:03 PM amounts to .5896180556 of a whole day.


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