You have maintained the condition records validity as from 01.03.2015 to 31.12.9999, then you should not expect that condition record will be valid for earlier than 01.03.2015.
If you want to use the calculation for earlier date than 01.03.2015, then of-course either you need to change the validity of the existing condition record or you need to create a new condition record which will be valid for the PO document date.