The zakat year begins on the date on which you were first in possessions of wealth above the nisab.
This will be your seed date, whenever it comes around you will have to calculate zakat, irrespective of any fluctuations in the amount of wealth in your possession.
The only situation in which your seed date will change is if you were to become totally bankrupt and lose all your assets and belongings. In this situation, your new seed date will begin when you are once again in possession of wealth above the nisab.
If you are not sure of your seed date, then estimate it to the best of your ability.
If after deducting these costs, his remaining wealth is less than the nisab then no zakat is payable.