Excel's Formula - sisman

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Part II: Using Functions in Your Formulas
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 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.
You may wonder about January 0, 1900. This non-date (which corresponds to date serial number
0) is actually used to represent times that are not associated with a particular day. This will
become clear later in this chapter.
To view a date serial number as a date, you must format the cell as a date. Use the Format Cells
dialog box (Number tab) to apply a date format.
Excel 2000 and later versions support dates from January 1, 1900, through December
31, 9999 (serial number = 2,958,465). Versions prior to Excel 2000 support a much
smaller range of dates: from January 1, 1900, through December 31, 2078 (serial number
= 65,380).
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 for Windows uses the 1900
date system, and Excel for Macintosh uses the 1904 date system. Excel for Windows supports
the 1904 date system for compatibility with Macintosh files. You can choose to use the 1904
date system from the Excel Options dialog box. (Choose File➜Options and navigate to the
When Calculating This Workbook section of the Advanced tab.) You cannot change the date
system if you use Excel for Macintosh.
Generally, you should use the default 1900 date system. And you should exercise caution if you
use two different date systems in workbooks that are linked. For example, assume that Book1 uses
the 1904 date system and contains the date 1/15/1999 in cell A1. Further assume that Book2 uses the
1900 date system and contains a link to cell A1 in Book1. Book2 will display the date as 1/14/1995.
Both workbooks use the same date serial number (34713), but they are interpreted differently.
One advantage to using the 1904 date system is that it enables you to display negative time values.
With the 1900 date system, a calculation that results in a negative time (for example, 4:00
PM–5:30 PM) cannot be displayed. When using the 1904 date system, the negative time displays
as –1:30: that is, a difference of one hour and 30 minutes.