When he goes to the social worker looking for his mother he says he was born in 1950. Later on in the movie he says he was born in 1951.
When John is riding around town showing his cartoons to various people, he passes one to a street guitarist. The guitarist takes the cartoon and says "I like the shading". In the next cut, the guitarist's hand rests on his shoulder as John closes his portfolio containing the cartoon.
John draws a cartoon about a Starbucks appearing in some guy's rectum, the joke being there are Starbucks everywhere, but Starbucks wasn't 'everywhere' until the 1990s/ 2000s, while this film takes place in the late 70s/ early 80s.
The movie takes place in the 1970s to mid 80s. Donnie refers to god as Chucky from the horror movie, but that movie [ Child's Play ] was not made until 1988.
When John and Annu meet in the bookstore, the time frame would appear to be the late 1970s or early 1980s. While the focus on the shelves is blurry, there are clearly recognizable covers, including Norton's 1995 reissue of A Clockwork Orange and Chris Cleave's 2008 novel Little Bee.
Dexter says "I came this close to reaching out to you". Nobody used the term "reach out" in that context at the time. It's a recent coinage of business babble.