Is Zakat obligatory on financial arrears retroactively for the past three years?

If these arrears resulted from a retroactive increase and were not received until after the mentioned period, then there is no Zakat (definition) on them upon receipt. Rather, Zakat becomes due after one year has passed from the date of their possession. If they are financial entitlements that could not be acquired, they are like usurped money, and their Zakat becomes obligatory upon their receipt. There is a difference of opinion regarding their Zakat: some say there is no Zakat on them upon receipt, while others say Zakat is due for the past years (which is the more cautious opinion), and yet others say Zakat is due for one year upon their receipt.