They could mean the same thing depending on the speaker. "I saw that film last January." "I saw that film in January." As the other commenters have mentioned

They could mean the same thing depending on the speaker. "I saw that film last January." "I saw that film in January." As the other commenters have mentioned, "last January" may mean January 2016 for them, whereas for others it's 2017. The second example should be a definite 2017, except you always have your occasional smart alec [0]. I would disagree with jasonkester--there is ambiguity here. [0] https://en.wiktionary.org/wiki/smart_aleck https://en.wiktionary.org/wiki/smart_aleck