Tax not calculating in miro, when i try to post it on back date.

HI expert

Tax not calculating in miro, when i try to post it on back date., i hve check the Condition validity its also ok. i did GRN on 3/4/12, try to miro on 3/4/12 system is not calculating tax on that date, when i am taking posting date on 4/4/12 system is calculating tax on miro.

Regard

Nabil