‘In a month’: this will happen in exactly one month, not before and not after. ‘Within a month’: this will happen between today and one month from today, and not after. ‘After a month’: this will happen sometime after a month has gone by, but not sooner, and it could be a long long time. However, when we use ‘month’ in that fashion, we don’t necessarily mean a calendar month, but (more commonly) any four week period of time counting from ‘this point’. If we mean ‘in a month’ to mean a specific month, we would specify which month (‘in June’, or ‘before the end of May’).