Q: If your Zakat date is after Ramadan but you wish to pay during Ramadan, how should your Zakat be calculated?
A: Once the zakatable wealth has reached the nissab value, a person can pay Zakat in advance for any amount of coming years. This is the opinion of the Hanafi school of jurisprudence. If your date is after Ramadan, and you wish to pay in Ramadan you can – however, when your actual Zakat date arrives, you must calculate what you owe as normal and then check whether what you paid in Ramadan was the correct amount, or if there is any left to pay.
As for the other schools of law, please see a short summary below:
The most famous opinion for the Malikis is that Zakat can be given only one month in advance.
According to the Shafi’i school of jurisprudence, Zakat can be paid in advance for up to only one lunar calendar year.
As for the Hanbalis, the correct opinion is that Zakat can be given in advance for a period of up to two years only.
And Allah knows best!