DAY function finds the day of the month of a given date.
|date||date||yes||The date for which to find the day|
The result will always be in the range 1 and 31.
In some cases you may want to accept only dates in the first half of the month. You can add a validation rule to ensure that the day of the month is the 15th or earlier:
DAY(reporting_date) <= 15