"This month is too short."
Translation:Ten miesiąc jest zbyt krótki.
Theoretically, they are mostly interchangeable, but it's not that easy. According to professor Bańko, you can always change "za" into "zbyt", but not necessarily the other way round. Anyway, as long as you are not completely fluent in Polish, I guess using one of them in a wrong way will not be that tragic ;)