What date is used to determine what is first in for FIFO purposes?

Customer has an inventory org that is not WMS-enabled, so using the standard picking Rules, set to FIFO.  What date is used to determine what is first in?  Is it the lot creation date or is it the creation date of the layer record in the quantity on hand table (mtl_onhand_quantities)?  If it is the latter, I would expect that later created lots could be used earlier than expected, if they have been moved into the subinventory/locator defined for picking ahead of lots created earlier into the receiving subinventory/locator.