This depends on when your Zakat becomes due. The date on which zakat should be calculated is what we call the Zakat anniversary date.
Every believer will have their own Zakat anniversary. To work out your date, it is exactly one lunar year after the date when your Zakatable wealth first reached or exceeded the value of the Nisab.
If you cannot remember the date your wealth first reached that level, then the date should be estimated.
If this is not possible, then a specific Islamic date should be selected arbitrarily and adhered to annually going forward.