By way of example,
point1) for the islamic dates as it pertains to 1430 and 1431 you must estimate as accurately as possible the amount you owe between the exact Islamic date of 1430 and 1431 and pay dispense of it.
point 2) You would calculate monies in this account and calculate the value of all your zakatable assets that was in your control once you become eligible (nisab.)
point3) If from a million dollars your zakatable assets dropped to 10 000 on the day zakah is due, then you would give zakah that would be owed on only the 10 000. Likewise, if you had 10 000 last year and this (Islamic date) year prior to the due date you got a million, then likewise, you would pay on the amount due on this one million.
May you be blessed, Ameen.
Allah Certainly knows Best.