Chainsaw Man takes place in a dark alternative world of the present-day world where Devils born through human fears are able to freely influence politics, war, and the ordinary way of living. Serialised by Tatsuki Fujimoto, the series mixes modern Japan with a metropolis with historical divergences drastically changing global history. To grasp the time of the events of the story, it is necessary to look not only at the direct mentions of the calendar but also at the alternate timeline that the series creates very thoughtfully.
Chainsaw Man mainly occurs in the period of 1997 to 1999 in this parallel history. The two principal sections of the story are distinctly framed by an in-world temporal gap, with Part 1 largely set in 1997 and Part 2 spanning over 1998 and even 1999. This period is supported with dialogue, seasonal points, and even allusions to world events that are exclusive to the series.

The series has a world that is highly divergent from the real-world history, way before the 1990s. World War II, nuclear weapons, AIDS, and even some historical ideas did not happen as they were washed out by the Chainsaw Devil. Because of this, Chainsaw Man lives in a late 20th-century world that looks like that of the 1990s but is functioning in radically different geopolitical circumstances.
Even with such divergences, the story always bases itself on an identifiable calendar. Heroes enjoy Christmas, school years, and respond to the seasons that coincide with the real-life Japan of the late 1990s. This information enables the reconstruction of the Chainsaw Man timeline with a high level of precision.

Part 1 of Chainsaw Man, known as the Public Safety Saga, opens with Denji as a devil hunter to pay off inherited yakuza debt in 1997. He then becomes a member of Tokyo Public Safety Devil Hunters and encounters Aki Hayakawa and Power, who are under the supervision of Makima. The Bat Devil, Eternity Devil, Katana Man, International Assassins, and the majority of the major arcs take place during this very calendar year.
Part 1 also ends in 1997, towards the close of the fall and the beginning of winter. The massive international attack made by the Gun Devil took place in September 1997 and claimed the lives of about 1.2 million individuals across the world in a few minutes. The ultimate battle with Makima occurs soon after Christmas, forcing the conclusion of Part 1 to the end of 1997.
A Timeline of Chainsaw Man: The Story Explanation in Part 1 and Part 2
After the events of 1997, Chainsaw Man experiences a time jump, which puts Part 2 in 1998 and 1999. Denji currently lives a less chaotic life as she attends school and raises Nayuta, the reincarnation of Control Devil. This change is the start of the Academy Saga, which literally alludes to the time since Makima was defeated.
The major events of Part 2 take place during the year 1998, such as the entry of Asa Mitaka as a new main character. The Falling Devil arc, where mass casualties on gravity are used to destroy sections of Tokyo occurs around March 1998. These incidents affirm that Chainsaw Man is definitely solidly in the late 1990s by now.

By 1999, it had built up to worldwide repercussions with the Chainsaw Man Church plotline and the arrival of the Famine Devil. Citations to the prophecy of Nostradamus, that the world will be destroyed in July 1999, give a definite chronological point of reference. The fact that the Soviet Union remained intact and there were Cold War-style tensions supports the alternate 1990s setting even more.
Finally, Chainsaw Man is set in the period between 1997 and 1999 in an alternative timeline of the late-20th century, where devils changed history and erased it. Part 1 is set nearly entirely in 1997, and Part 2 covers 1998 with a jump to 1999 as the stakes of the world are raised. The story bases its horror and action on a particular historical window, which increases its realism and intensifies the effect of its changing world.