Do both mean 'after'? I once heard that 'nach' has a specific start point while 'in' doesn't, but I don't really understand what that really means.
I came across this example :
Ich plane, mein Buch bis in zwei Monaten zu beenden
If I was writing this sentence, I would have never included 'bis'
Secondly, whenever I read it, I immediately translate 'in' to 'within' 2 months, not after 2 months I am really confused regarding the difference between those prepositions ( nach vs in ) and the reason 'bis' was added