They're different forms of the word depending on number and case. The former should be obvious, the latter is (as always) determined by the grammatical context in a sentence. Jahr, Monat, Tag: nominative, dative and accusative singular Jahre, Monate, Tage: nominative, genitive and accusative plural Jahren, Monaten, Tagen: dative plural

It pretends on the preposition you use whether to take the Akkusativ oder Dativ, but this very theoritical and deeper stuff, i will find try to find out a good explaination later. Hard to explain in a few seconds(even i dont know the "rules")