When the provider created this checkout, in the ledger's canonical ISO 8601 form (YYYY-MM-DDTHH:mm:ss.sssZ), or undefined if the provider didn't supply a usable timestamp. Each provider normalises its own format (see toCanonicalIso) so this is safe to use directly as a ledger occurredAt. It is the customer's business time, so a payment processed late — a delayed webhook, an old redirect, a stale retry — is still recognised on the day it was paid, not the day we happened to process it.