Professor layton dating game
For Dai Gyakuten Saiban: Naruhodō Ryūnosuke no Bōken, the basis for its timeline is the date of birth of the character Sōseki Natsume, who is based on the real life author of the same name.
As far as the Bonus Episodes are concerned, they are intended to give more of a "Behind the Scenes" perspective on the game and are not considered canon.
This has led to some inconsistencies in character birthdates.
The timeline for this page treats the "present" timeline of an individual game as a "year" (much like school terms that straddle the new year), which resolves some of these issues. Since the organizer profiles in Ace Attorney Investigations: Miles Edgeworth and Gyakuten Kenji 2 fit with corresponding court record profiles in Phoenix Wright: Ace Attorney: Trials and Tribulations, the 2018-2019 "year" used in Trials and Tribulations is used for the purpose of birth years in these games. Please note that some of these dates are extrapolations (e.g., "two weeks after Phoenix Wright's disbarment" becomes "May 3") and may not be exactly correct.
Nonetheless, it's unlikely that the "true" dates will ever be pinned down.
Additionally, while it is noted above that the basis for placing The Adventure of the Clouded Kokoro in 1900 is based on the age of Sōseki Natsume, the real Sōseki Natsume was only present in the UK from 1901 to 1903. As he returned to Japan in January 1903, yet returns in the game between The Memoirs of the Clouded Kokoro (February) and The Adventure of the Unspeakable Story (April), plus the fact that he would be 36 in 1903, the real-life dates are rendered irreconcilable with the dates in-game.