Zakat from Past Years

If a person is trying to calculate the zakat they owed on past years (because they were unaware that it was obligatory on them) then,
1) at what point in every year should they calculate the zakat?
2) what if their account fluctuated over the course of the year?
( they may have had less the minimum amount?) should they look at past monthly bank statements?
3) what if the reason for that fluctuation was various expenses such> as clothing, other small miscellaneous purchases (beyond what is needed), and tuition?
4) what is the minimum amount that must be had to pay zakat?
5) what if they have a loan, should they delay paying the zakat until the loan is paid?
6) how is the zakat al-fitr calculated for those past years?

Al-Salam alaykum

1. The Zakah-date of a person is the date upon which he/she became the owner of Nisab in the condition that they were baligh (mature). Since you do not know the exact date you will have to estimate it to the best of your ability.

2. The condition is that the Nisab amount must be reached at the beginning and end of the Zakah year even if it fluctuates to lower than this during the course of the year.

3. The fluctuation does not affect the Zakah date unless the assets reach zero.

4. This is calculated according to the country. Please ask your local ulama body.

5. If by loan you mean you are indebted then the liability of the debt will be subtracted from the total zakatable assets and zakah will be paid on the net value.

6. Zakah al-Fitr will have to be paid for the past years by determining the value of half a sa` (about 2 kg) of wheat on the day of `Id of those past years. For each year the relevant value must be given as Zakah al-Fitr.

Abu Usamah