previous years

Q: My father saves enough money and my mother has jewellery. I know the nisaab for savings, gold and silver. As per the criteria, Zakat is wajib on my parents. Till now, in their entire life they didn't pay zakat. They did toubah (repented) for not paying it. Now how do they pay zakah for all the previous years because they don't know when they became sahib-e-nisaab.

Also determining the savings of my father for all the previous years is quit complicated. How much should they start giving from this year?

Q: If one has not payed zakaat for previous years and now wishes to do so, will they calculate when they became saahib nisab based on todays nisab or nisab of that particular day?

Q: I didn't know that I was eligible to pay zakaat. I was just informed recently thanks to your answer to a previous question. Now I have had money above nisaab for two years. Should I pay zakat immediately or can I wait till ramadhan and pay for the two years together?