The following is a timeline of events in the Twin Peaks fictional chronology, covering all officially released material.
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Dating conventions
Twin Peaks (1990-91)
Although the show contains many inconsistencies, it is commonly agreed that the events take place in February and March 1989.
In the Pilot, Dale states into his tape recorder that the date is February 24, and Ronette's medical chart in episode 8 confirms that her hospital admission on that date is February 24, 1989. This year is given in subsequent releases, including Twin Peaks: Fire Walk with Me and The Secret History of Twin Peaks. Occasionally, incidental dates visible onscreen differ – for instance, Emory Battis' book in episode 6 describes events up to February 25, 1990 when the date at the time should be March 2, 1989.
Before the original TV broadcast of Episode 24, a short vignette featuring Dale Cooper summarizing recent episodes uses the episode's airdate (Thursday, March 28, 1990) rather than the in-story date.
Twin Peaks: Fire Walk with Me
The film establishes Laura's murder to be on a Thursday to Friday night.
The Secret Diary of Laura Palmer
The novel was written to place Laura's death after October 31, 1989 with her birthday being July 22, 1972, contrary to the TV show which places her death on February 24, 1989 at age 17.
To compensate, the dates for all events that do not otherwise conflict with the series or Fire Walk with Me are placed one year prior.
My Life, My Tapes
The account of Caroline Earle's murder by her husband is set in 1979, contradicting dialogue in Episode 17 that places it four years prior (c. 1985). To account for this, all events in Part 5, Chapter 2 (April 1978) through Part 6, Chapter 1 (July 1980) are advanced forward six years (1984 – 1986).
Twin Peaks (2017)
The current dating scheme originates from the arrest date seen on the report for Cooper's doppelganger, which indicates that his accident and arrest (in Part 3) took place on 9/22. In Part 9, the date in Twin Peaks and Buckhorn is established to be September 29. Whereas the events surrounding Dale Cooper in Las Vegas have covered only a few consecutive days, for the other subplots this requires a cumulative gap of about five days between the FBI's first interview with Cooper in prison (Part 4 / September 23) and Diane Evans's visit (Part 7 / September 28).
The current year is not stated. If William Hastings' interrogation on a Saturday (Part 1) is taken to be two days before September 22, this would point to 2014 as the real September 22, 2014 was a Saturday. However, The Secret History of Twin Peaks, possibly set before the series, is set in 2016. The Search for the Zone website indicates that Hastings last updated the blog in November 2015.
It is unclear to what degree the events taking place in and around Twin Peaks are concurrent with those involving Dale Cooper and the doppelganger, or even other Twin Peaks-set scenes. For example, the Great Northern Hotel key mailed by Jade in Part 5 arrives in town at some point during Part 7, which (if set contemporaneously) would require a transit time of less than a day. Andy Brennan's watch in Part 7 shows that the date is the 10th, possibly October 10.