When Does “Watchmen” (1986) Take Place?
“Watchmen” is a political science fiction superhero comic-book written by Alan Moore that premiered May 13th, 1986. So when is "Watchmen" set?
It takes place on an alternate timeline of the fictional DC Comics Universe in the year:
We know this because Rorschach’s journal excerpts are used as a narration device and are clearly dated from October of 1985 AD in issue #1 to November of 1985 AD in issue #10. So it is most likely October of 1985 AD at the beginning.

1985 AD
We know this because Rorschach’s journal excerpts are used as a narration device and are clearly dated from October of 1985 AD in issue #1 to November of 1985 AD in issue #10. So it is most likely October of 1985 AD at the beginning.
Then, in the first pages of the final issue #12, we can see piles of crowds attending a concert at Madison Square Garden behind a sign that reads "November 2nd" with a sticker on it that says "SOLD OUT". Four pages later, Doctor Manhattan says "Midnight, November second." So it must be November at the end of the main action.
In the second-to-last scene of the final issue, it is clearly Christmastime (therefore December) and a final two-page epilogue with a publisher and his assistant could be taking place in the next year but it's unclear how much time has passed since the Christmas scene.
It was adapted to film in 2009.
It was adapted to film in 2009.