Field notes · 12 May 2026
Where proration quietly invents revenue
Mid-cycle upgrades look simple until the day-count method in the contract disagrees with the billing calendar.
Proration clauses often say “based on the number of days remaining in the billing period.” Billing systems often say “divide by thirty.” Those two sentences produce different invoices the moment a customer upgrades on the 31st of a long month — or on the 28th of February.
When we test mid-cycle changes, we rebuild the expected charge from the clause, not from the system’s output. The working file lists:
- The list price before and after the change
- The effective date on the amendment
- The day-count method named in the order form
- The amount actually invoiced
Exceptions cluster in three places: upgrades that ignore unused days on the old plan, downgrades that credit a full month instead of a partial one, and promotional free days that overlap a paid period. None of these require exotic tools — they require reading the amendment and doing the arithmetic on paper once.
If your close process only checks that “something prorated,” ask for the day-count method next time. It is the cheapest control you can add before an external reviewer arrives.