Paying Missed Zakaat Nisaab

Assalamu Alaykum. How are you Mufti Saheb? Does the Nisaab amount fluctuate daily, monthly or annually?
If so, how can i find out the Nisaab amount for specific dates over the last 5 years?
Does anyone keep a record of the Nisaab amounts over the years?

i have 3 sons who have savings accounts. The balance has surpassed the Nisaab amount five years ago in all their accounts.
please advise.