Date of Zakaat

Q: I want to pay my zakaat and I do not know the exact date. It was between February 18 and end of November when I was 9 years old. I am inclined towards more towards November that I became baligh. I do not know which date to choose. Which date must I choose?

Zakaat on Cash Kept in Trust By The Bank For the Seller

Q: One is in the process of purchasing a property and pays the deposit to a bank & not the seller of the property. The bank then issues a guarantee (to the sum of the cash deposited) in favour of the seller of the property. The guarantees will be paid over to the seller once the property is transferred into the purchaser’s name.
At one’s Zakaat date the property has not yet been transferred and the cash is still at the bank in the form of a guarantee. Is such cash then Zakaatable?

Calculation of Nisaab of Silver

Q: In the following article on your website: http://alhaadi.org.za/articles/qurbani/668-who-is-required-to-perform-q… is mentioned that "Every adult Muslim, male or female, who owns 613.35 grams of silver or its equivalent in money, personal ornaments, stock-in-trade or any other form of wealth which is surplus to his basic needs, is under an obligation to offer a Qurbani."
Can you please give the reference of Hadith or other authentic books as to how the figure of 613.35 has been deduced?