If there is a monthly trial period for your customers, the period will end on the same day of the month. When the customer started the trial e.g. on the 01/10/17 the trial will run until the 02/10/17. If the trial starts at the end of the month and the following month has got less days, the trial will end on the last day of the following month. A trial, which started on 1/31/17 will e.g. end on 02/28/17.