Belinda the GP CSI provide some great information on when checks on the fly post to GL in her post Check on the Fly, What’s my Posting Date
Dynamics GP Check on the Fly, What’s my Posting Date?
General Ledger & FinancialsPayables & Receivables
Archived post from 2010. Dynamics GP and the links it points to have changed since; check current documentation before relying on version-specific steps.