Is zakat obligatory on money a husband gives his wife as a gift or allowance, and on the monthly salary if it reaches the nisab? And is it permissible to set a specific date for zakat if it is not known when the money reached the nisab?

The money gifted to the wife and the salary she owns are subject to zakat if they reach the nisab and a full lunar year has passed over them. The zakat payer may designate a single time for paying the zakat on all of their renewable wealth. They may pay the zakat in advance, but it is not permissible to delay it. If one is ignorant of the exact time the nisab was reached, they must investigate and act according to their best judgment.