Stricter date format constraints

[kylef]

This adds tests for the date format to ensure that the month day-length is permitted following the day restrictions, there is 2 tests for each month (one at last day and one negative at day after). February has two sets of tests for normal and leap years.

5.7. Restrictions

The grammar element date-mday represents the day number within the
current month. The maximum value varies based on the month and year
as follows:

  Month Number  Month/Year           Maximum value of date-mday
  ------------  ----------           --------------------------
  01            January              31
  02            February, normal     28
  02            February, leap year  29
  03            March                31
  04            April                30
  05            May                  31
  06            June                 30
  07            July                 31
  08            August               31
  09            September            30
  10            October              31
  11            November             30
  12            December             31

Appendix C contains sample C code to determine if a year is a leap
year.

There contains a test for invalid months too.

[Julian]

Thank you! Think these all seem great. Sincere apologies for the delay.