How do I set up a standard cost that's effective from a past date?

You can create a cost scenario with an effective date in the past and create standard costs for this scenario.

You can then roll up the costs and publish them, as long as the newly published costs don't conflict with any already published costs for the corresponding items.

Note: When you publish the cost of an item for the first time in costing, the effective start date is 01-01-1900, irrespective of the effective date of cost scenario. This ensures that you don't have to manually backdate the cost to process any transactions.