If a person becomes a Muslim after the month of Ramadan has passed and they were not Muslim during that time, they are not required to make up the missed fasts from before their conversion. The obligation to fast during Ramadan applies to individuals who are Muslims at the time of fasting.

However, once a person embraces Islam, they are expected to observe fasting during Ramadan as it becomes obligatory for them. They would fast in the upcoming Ramadan and in all subsequent Ramadans as a part of their religious practice.

So, to clarify, a revert (someone who has converted to Islam) is not obligated to make up missed fasts from the time before they were Muslim, but they are required to fast during Ramadan as a practicing Muslim.

But, if the person was a Muslim and went on Riddah (disbelieve after embracing Islam), then later reverted back to Islam, It is mandatory for him to observe the missed fasting during his out period also.