IBM 5150 Manual To Operations page 82

For example, suppose the current date is June 8, 1982.
Type this:
6-8-82
or this:
6/8/82
or this:
6-08-82
or this: 6/08/82
or this:
06-08-82
or this: 06/08/8!
or this: 06/8182
DOS checks the date that is typed. If the date that is
typed does not check out, DOS displays this message:
Invalid date
Enter today's date (m-d-y):_
These are some dates that would not check out:
Date
Reason
060882
No dashes or slashes
APR-02-80 Letters instead of numbers
13-08-83
Month too big
9-32-82
Day too big
10 15 82
Spaces, not slashes or dashes
If DOS tells you that the date is invalid, try again.
DOS 3-8