If there is a monthly trial period for your customers, the period will end on the same day of the next month. When the customer started the trial e.g. on the 10.01.2017 the trial will run until the 10.02.2017. 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 31.01.2017 will e.g. end on 28.02.2017.