If you select an invalid date (such as the 31st of April), the last day of the month is used. If, for example, you schedule a job to run on the 15th and the 31st of every month, your job would run on the 15th day and the last day of each month. If you schedule a job to run on the 28th and the 30th of every month, the job would run once in February of a non-leap year, and twice in February of a leap year (two jobs cannot be scheduled on the same day).