Excel looks at the date and does its best to decide if the date is in the 1900's
or the 2000's.
Lots of people are still entering dates in the 1990's. If you don't want excel
to guess, then you should enter the 4 digit year.
This is taken from xl2003's help for: About dates and date systems
How Excel interprets two-digit years
To ensure that year values are interpreted as you intended, type year values as
four digits (2001, rather than 01). By entering four digits for the years, Excel
won't interpret the century for you.
For Microsoft Windows 2000 or later
If you are using Microsoft Windows 2000 or later, the Regional Options in
Windows Control Panel controls how Excel interprets two-digit years.
For dates entered as text values
When you enter a date as a text value, Excel interprets the year as follows:
00 through 29 Excel interprets the two-digit year values 00 through 29 as the
years 2000 through 2029. For example, if you type the date 5/28/19, Excel
assumes the date is May 28, 2019.
30 through 99 Excel interprets the two-digit year values 30 through 99 as the
years 1930 through 1999. For example, if you type the date 5/28/98, Excel
assumes the date is May 28, 1998.
