Is zakat obligatory on money saved from one's monthly salary, and should this money be zakatable retroactively if it's impossible to calculate it, or should it be zakatable after a full year has passed from the date of becoming aware of the obligation of zakat?

Zakat is obligatory on the saved amount from the salary if it reaches the nisab (which is equivalent to 85 grams of gold or 595 grams of silver) and a complete lunar year has passed over it. The obligatory zakat amount is one-quarter of one-tenth (2.5%). If zakat became due in past years and was not paid due to lack of knowledge, it does not lapse; rather, it must be paid immediately upon gaining knowledge and ability. One must strive to estimate the amount of wealth for each past year and pay zakat on it. Whoever delays zakat without a valid excuse commits a sin, and he must pay it for all overdue years. He should act according to his best estimate in assessing the wealth and number of years if he is uncertain.