Is it permissible to make up the days that I missed in Ramadan due to surgery and treatment after the next Ramadan, or must they be made up before the coming of the next Ramadan?

The missed fasts of Ramadan must be made up before the advent of the next Ramadan for those who are able to fast, based on the Almighty's saying: "But whoever among you is ill or on a journey [during them] - then [he must make up] an equal number of other days." It is not permissible to delay the making up of these fasts until after the next Ramadan without a legitimate excuse. Whoever delays making up the fasts without an excuse until the next Ramadan begins, must make them up along with paying an expiation for the delay.