Billing timing decides which period an invoice covers. It's set org-wide in Settings → Finance → Invoices and overridable per customer in the Edit Customer modal.
Arrears — the default
The invoice is cut at the end of the billing period and covers the period that just finished, so everything completed in the window is on it. Every customer created before this setting existed is on arrears, and their behavior is unchanged.
Advance — billing ahead
The invoice is cut *before* the period starts and covers the period ahead. Two reasons to use it:
What's on an advance invoice
An advance invoice is a hybrid: the upcoming period's recurring charges, plus a clearly labeled true-up for usage from the period that just ended — extra pulls, overages, anything that couldn't have been known when the invoice was cut.
Due dates work differently
Advance invoices skip the "due at period end" floor that arrears invoices use. Payment terms run from the send date, so Net-15 means 15 days from when it was sent — not 15 days after a period that hasn't started yet.
Two things worth knowing
The timing is stamped on the invoice. It's recorded when the invoice generates, not looked up later. Changing a customer from arrears to advance never re-dates invoices you've already issued — so if an invoice looks like it covers the wrong month, check the timing on the invoice itself rather than on the customer.
Flipping a customer mid-cycle produces a catch-up invoice. Switching to advance bills the current in-flight period as well as the upcoming one, because otherwise the period already running would never be billed at all. Expect two invoices on the first run after the change, then a steady rhythm.
One restriction
A customer billed from completed work can't be on advance. At the start of a period nothing has been completed, so the invoice would be empty — PortaPro keeps those customers on arrears regardless of the setting.
No mid-cycle proration
Neither timing prorates a partial month mid-cycle, and advance billing doesn't create deferred revenue entries. This matches how QuickBooks Online and Xero handle the same situation, so your books stay reconcilable.