For example, in 2002 you owned 100 gm gold, then 2.50 gm gold was the due Zakah; so, deduct the same quantity from it. Again in 2003, deduct 2.5% of total 97.5 gm as Zakah. Repeat the same process in 2004, 2005, 2006, 2007 and 2008. You shall have to pay the total quantity of gold in Zakah. Also, you can pay the price of the gold as per the market rate of every concerning year. It is to be noted that the Zakah will be calculated according to the lunar calendar; therefore it is advisable to you to pay some extra amount as Zakah from 2002-2008 and in future calculate it as per the lunar calendar.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband