The dates on the Inventory movement reports are the dates the transactions were entered under, not the dates they were actually made on. If a transaction is backdated, then the inventory level on the report may appear to go negative, when there was sufficient inventory on hand to process the transaction at the time it was made.
The only Quantity number you can reconcile against is the Quantity on Hand "now". Every other number on the reports is calculated backwards from the current quantity on hand.
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Andrew Bates