How to Insert the record only once in the current Month?

Question

how to insert the record into the database by date/time .The same id will not inserted another time until a month over..

For example, take salary detail if we inserted employee by the employee ID with date/time in the database about the salary detail ..the same person Id could not inserted again until the one month....If one month over we can insert the same faculty details again...

Thanks in Advance