Part II: Working with Formulas and Functions
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 concept 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.
Choose Your Date System: 1900 or 1904
Excel 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 pre-2011 versions
of Excel for Mac use the 1904 date system.
Excel for Windows supports the 1904 date system for compatibility with older Mac 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 When Calculating This Workbook section.) 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. 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).
Entering dates
You can enter a date directly as a serial number (if you know the serial number) and then
format it as a date. More often, you enter a date by 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 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, 2018, into a cell, you can enter the date by
typing June 18, 2018 (or any of several different date formats). Excel interprets your
entry and stores the value 43269, the date serial number for that date. It also applies the
default date format, so the cell contents may not appear exactly as you typed them.
