The extrapolation of the contracts will determine if your orders are created with a start and end-date similar to a calendar month. There are two options:
Not extrapolating a contract. Let a contract run from it start date until exactly a month later
An example would be a monthly order running from 15 November until 14 December. And the following order from 15 December until 15 January.
Extrapolating a contract. Create an initial order for the period from the start date to the next month and keep the orders in sync with calendar months
An example would be a monthly order running from 15 November until 30 November and an order for 1 December until 31 December. The following order would be 1 January until 31 January.
The two option would be graphically represented in the following form:
When an extrapolated contract is terminated, the last order will be an order that contains the rest of the month and is calculated on the number of days there are still left after subtracting all the remaining months in the contract.