Microsoft Office

Part II
Working with Formulas and Functions
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 the date system for the active workbook in the Advanced section of the Excel Options dialog
box. (It’s in the subsection titled When Calculating This Workbook.) You can’t 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 together. For example, assume that Book1 uses the 1904 date
system and contains the date 1/15/1999 in cell A1. Assume that Book2 uses the 1900 date system and contains
a link to cell A1 in Book1. Book2 displays the date as 1/14/1995. Both workbooks use the same date
serial number (34713), but they’re 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 1 hour
and 30 minutes).
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 “Entering times”).
To view a date serial number as a date, you must format the cell as a date. Choose Home ➪ Number ➪
Number Format. This drop-down control provides you with two date formats. To select from additional
date formats, see “Formatting dates and times,” later in this chapter.
Entering dates
You can enter a date directly as a serial number (if you know it), but more often, you enter a date using any
of several recognized date formats. Excel automatically converts your entry into the corresponding date
serial number (which it uses for calculations), and it also applies the default date format to the cell so that it
displays as an actual date rather than as a cryptic serial number.
For example, if you need to enter June 18, 2007, you can simply enter the date by typing June 18, 2007
(or any of several different date formats). Excel interprets your entry and stores the value 39251, the date
serial number for that date. It also applies the default date format so that the cell contents may not appear
exactly as you typed them.
Depending on your regional settings, entering a date in a format, such as June 18, 2007, may
be interpreted as a text string. In such a case, you’d need to enter the date in a format that corresponds
to your regional settings, such as 18 June, 2007.
NOTE
When you activate a cell that contains a date, the Formula bar shows the cell contents formatted by using
the default date format — which corresponds to your system’s short date format. The Formula bar doesn’t
display the date’s serial number. If you need to find out the serial number for a particular date, format the
cell using a nondate number format.
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