Warning! Spoilers ahead for Joker!
There aren’t many Easter eggs or direct comic book references in Joker, but there’s one cinematic connection to an existing Batman film that even the editor of Joker didn’t catch.
During the riots that envelop Gotham City late in the film, the Waynes exit a movie theater before they’re gunned down in an alley. While the year Joker is set in is never explicitly stated, all four movies playing at the theater the Waynes leave were released in 1981: Blow Out, Zorro the Gay Blade, Arthur, and Excalibur.
While the Waynes going to see a Zorro movie has long been part of Batman’s origin lore (the swashbuckler being a direct influence on the character), Joker isn’t the only DC film to set the murder of Thomas and Martha Wayne in 1981 near a cinema where one of the aforementioned films was playing.
Source: IGN.com Did You Catch This BvS Easter Egg in Joker?