diary-abbreviated-year-flag

diary-abbreviated-year-flag is a variable defined in `diary-lib.el'.
Its value is

t

Documentation:
Interpret a two-digit year DD in a diary entry as either 19DD or 20DD.
This applies to the Gregorian, Hebrew, Islamic, and Bahá'í calendars.
When the current century is added to a two-digit year, if the result
is more than 50 years in the future, the previous century is assumed.
If the result is more than 50 years in the past, the next century is assumed.
If this variable is nil, years must be written in full.

You can customize this variable.