Hi Kate,

Backdating an Item Valuation report is meant to calculate the qty and total cost of items as of that date, using the current figures and the Transaction History records for the difference. From that, the unit cost is further calculated. If History is not complete or if an OE Day End has not been performed, this process will not give accurate results.

If there are particular examples as you have given that give unexpected results, we can take a look.
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Regards,
Softrak Tech Support