TL;DRThe day of the month a schedule's recurring invoices are dated.
Unless the contract states an invoice day, the billing day is the day-of-month of the subscription's start — capped at the 28th so every month has it. A contract starting on the 15th bills on the 15th, not the 1st.
When a schedule is activated part-way through a cycle, the first invoice covers the shortened window up to the next billing day and is priced by calendar units, not by a days-over-days ratio.
Where it happens in Sonic
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See it on your contracts
A walkthrough on the agreements and files you actually bill from — not a slide deck.