Is Zakat to be paid in the month of Jumada al-Akhira, which is when the nisab is reached, or in the month of Ramadan as was done previously? And does delaying the payment of Zakat for the past two years necessitate making up for it or estimating the value of Zakat for the months of delay, knowing that Zakat on all wealth was paid in the month of Ramadan?

Zakat (definition) must be disbursed immediately once the wealth reaches the nisab (definition) and a hawl (full lunar year) has passed over it. Delaying it without a valid excuse is a sin, unless there is a legitimate religious reason such as the absence of poor people, or if the delay is very brief. So, if the hawl for your wealth is in Jumada al-Akhirah (definition), do not delay the zakat until Ramadan (definition); rather, it must be disbursed in Jumada al-Akhirah. As for the previous questioner’s delay of his zakat out of ignorance, there is no sin upon him, and he must disburse it this year at its proper time.