When Does the Film "Joker" (2019) Take Place?
"Joker" is a crime film written by Todd Phillips with Scott Silver [loosely and partially adapting "Batman: The Killing Joke" (1988) by Alan Moore] that released October 3rd, 2019. So when is "Joker" set? 
 
 It takes place on an alternate timeline of the fictional DC Films Universe in the year:
It takes place on an alternate timeline of the fictional DC Films Universe in the year: 
 
We know this because there is a major garbage strike and, while Gotham City is a fictional place, it's real-world basis has long been accepted to be partially the American Eastern urban centers Chicago/Jersey City/New York City, and New York City does have a history of sanitation strikes. The real-world New York City Department of Sanitation garbage strike of 1981 AD began on December 1st, 1981 AD and ended December 17th, 1981 AD (though there was also one in February of 1968 AD and in July of 1975 AD) so the movie probably takes place in December of 1981 AD.

1981 AD
We know this because there is a major garbage strike and, while Gotham City is a fictional place, it's real-world basis has long been accepted to be partially the American Eastern urban centers Chicago/Jersey City/New York City, and New York City does have a history of sanitation strikes. The real-world New York City Department of Sanitation garbage strike of 1981 AD began on December 1st, 1981 AD and ended December 17th, 1981 AD (though there was also one in February of 1968 AD and in July of 1975 AD) so the movie probably takes place in December of 1981 AD.
Furthermore, when Arthur Fleck passes by an "ARCADE" a poster on the wall advertises "PONG". "PONG" is an early video game released on November 29th, 1972 AD in the real world. So it must be after November of 1972 AD. This certainly means it can't be during the NYC garbage strike of 1968 AD. 
At the end of the narrative, the Wayne family walks under a movie theater marquee that reads: "NOW SHOWING  BLOW OUT  ZORRO THE GAY BLADE" and walk past a poster reading "Excalibur" and "Arthur". The films "Blow Out" (1981), "Zorro the Gay Blade" (1981), "Excalibur" (1981), "Arthur" (1981) all released in April through July of 1981 AD in the real world, which means it must be during or very soon after 1981 AD.  This definitely means it can't be during the NYC garbage strike in 1975 AD. 
More specifically, at the very beginning of the story a radio news broadcast declares, "Good morning. It's forty-two degrees at 10:30 on this Thursday, October 15th. I'm Stan L. Brooks and here's what's happening. It's day 18 of the garbage strike"... Now, "It's day 18 of the garbage strike"... of 1981 AD would make the story start on December 18th, 1981 AD (a day after the real-world 1981 AD NYC garbage strike ended) HOWEVER the electronic voice tells us it is clearly October of 1981 AD in this early scene. The garbage strike must have started sooner in the fictional world (diegesis) of "Joker", maybe even in the wider DC Multiverse. 

