Dates are complex enough without times and timezones. This module may
be used to create simple date objects. It handles:
Validation:
Reject 1999-02-29 but accept 2000-02-29.
Interval arithmetic:
How many days were between two given dates? What date comes N days
after today?
Day-of-week calculation:
What day of the week is a given date?
It does NOT deal with hours, minutes, seconds, and time zones.
