Monthly access means that the access will expire on the same day and time in the next month. So 1 month is not equal to 30 days.
If the user activated the card on 9th, February, it will be expired on 9th, March. In this case the access will last less than 30 days.
In case the access would be expired on a day which doesn’t exist (for example the user bought an access on 31th, January, but there is no 31t, February), the access will expire on the last day of the next month (in this example on 29th, February).