Is Zakaat retro-actively applicable on tax refund?

I paid zakaat in Ramazan which was November 2001.However I will get a tax refund for the year 2001 in april 2002 insha allah. Now this money was actually my money however not in my possession until the government pays me back on filing my returns. This generally happens if there is an overpayment of Tax (during 2001 in this case) or tax laws change. Is zakaat due for the year 2001 on the refunded money which I got in 2002 or it is due only for 2002. It is not generally known , exactly how much money would be refunded unless we file our taxes in US which is before April 15th. After filing taxes either we get a refund or we have to pay more to the government.