You want to make sure that a date or time a user entered is valid. For example, you want to ensure that a user hasn't attempted to schedule an event for the 45th of August or provided a credit card that has already expired.
If your form provides month, day, and year as separate elements, plug those values into checkdate( ), as in Example 9-16. This tells you whether or not the month, day, and year are valid.
Checking a particular date
To check that a date is before or after a particular value, convert the user-supplied values to a timestamp, compute the timestamp for the threshhold date, and compare the two. Example 9-17 checks that the supplied credit card expiration month and year are after the current month.
Checking credit card expiration
checkdate( ) is handy because it knows about leap year and how many days are in each month, saving you from tedious comparisons of each component of the date. For range validations'making sure a date or time is before, after, or between other dates or times'it's easiest to work with epoch timestamps.
9.8.4. See Also
Chapter 3 discusses the finer points of date and time handling.