Zakaat on money received during the year

If I have 20 lakh rupees in my account at the start of the year and after completion of one year this amount is increased to 30 lakh over the last twelve months gradually on monthly savings from my source of income. To my knowledge if an amount is in my possession for one year constantly, the zakat is due on that amount, therefore I assume that I should calculate my zakat on 20 lakh because the increase in my amount is month by month and full one year has not been passed over this amount. Am I correct or I should calculate my zakat on 30 lakh?