I'm working on an invoicing system for a site. There are 3 different service plans available: Download 1 product per day Download 1 per week. Download 1 per month. There needs to be a single invoice per month that calculates the total for the plan(s) the user has. Some users have 3 or 4 different plans on their account, added at different times. For example first they order "1 per day" on the 10th, then they order "1 per month" for something else on the 17th. They they order "1 per week" for something else on the 22nd. Their monthly invoice date for their account would be the 10th. How do I put all of them in to one invoice each month? I don't want to send 3 separate invoices each month. I can't get my head around the overlapping times.