In a large project management application, there is a category of resources which are managed in a special way. During the planning phase you have an activity with a planned start date and a planned finish date and a monthly sequence of estimated effort values for one or more resources. A period always refers to a real calendar month with period 1 being the month of the planned start date. But the first period and last period may not use up a full month.
This problem is related to a reservation system where customers book a service and pay it in one or more transactions which may happen on different dates.