Every placement lands somewhere inside a billing cycle. The question is what the first invoice says. Operators handle it three ways: bill the full cycle and hope nobody argues, bill nothing until the next cycle starts, or prorate. The first loses customers, the second loses money, and the third is five minutes of arithmetic that the software should do for you.
The inclusive-day rule
Take a 28-day cycle running August 16 to September 12, and a unit delivered on August 30. Count the days the unit was on site inside the cycle, counting both ends: August 30 through September 12 is 14 days. The prorated charge is 14/28 of the cycle rate. At $120 per cycle, that is $60.
Counting both ends matters. "August 30 to September 12" is 13 days if you subtract the dates and 14 if you count them. The unit was there on the 30th and on the 12th, so it is 14. Pick the inclusive rule and write it down, because a customer with units landing every week will notice a one-day inconsistency.
Monthly cycles prorate against the month
If the customer is on a calendar month, the denominator is that month's length. A unit placed on March 20 bills 12/31 of the monthly rate; one placed on February 20 bills 9/28. This is the one place where the 28-versus-30 difference shows up inside a single invoice, and it is also why a rate card row should be on the same period as the customer's cycle. A monthly row on a 28-day customer prorates against the calendar month while every other line is on the cycle, and the invoice looks wrong even when the total is right.
The Bills From date, not the delivery job
The date that drives proration is the date billing starts, which is usually the delivery date but not always. A unit dropped the Friday before a Monday start, at the customer's request, bills from Monday. A unit the crew was supposed to deliver Monday and actually delivered Wednesday bills from Wednesday. Put that date on the rate card row, call it Bills From, and let the invoice read it. Never work it out from the job.
A blank Bills From date is the most common cause of a wrong first invoice: the engine has nothing to prorate from, so it bills the full cycle. Flag blanks before the cycle closes. In PortaPro that is one of the Billing Health checks, and it only fires until the first invoice is out, because after that every cycle is a full one anyway.
What the line should say
The invoice line for a prorated unit should carry the dates and the fraction in words: "Standard unit (prorated Aug 30 to Sep 12: 14 of 28 days)". Two reasons. The customer does not call. And in six months, when someone asks why that invoice was $60, the answer is on the invoice, not in a spreadsheet nobody can find. A line without that tail was either a full cycle or hand-edited, which is worth knowing too.
Worked example: a builder, three lots, three dates
Ridgeline Homes takes one unit on lot 3 on August 18, two units on lot 7 on August 30, and one on lot 11 on September 10. Cycle is August 16 to September 12 at $120 per unit per cycle.
- Lot 3: 26 of 28 days, $111.43.
- Lot 7: 14 of 28 days × 2 units, $120.00.
- Lot 11: 3 of 28 days, $12.86.
First invoice $244.29. Next cycle, all four units bill $480.00 with no proration line. If the builder wants one invoice per lot, the same math produces three invoices on the same day; see per-site invoicing.
When not to prorate
Events. A weekend festival is a flat rental, not a cycle, and prorating a three-day rental makes no sense. Keep event rentals as one-off quotes with a start and end date, and keep proration for the rate card book. The two should never share an invoice line.
