At the moment these don't really mean anything, but I thought I'd compile a list of dates and times that have been mentioned so far. More for my own curiosity than anything else (I have a thing for continuity timelines.) These are all with the (likely) assumption that the show takes place in 2012.
- 1837 - Irrational Treasure - Quentin Trembley wins Presidential Election.
- 1862 - The Time Traveler's Pig - Mabel and Dipper traveled back in time by 150 years to this point. Fertilia and her family travel on the Oregon Trail.
- May 2, 1995 - The Inconveniencing - If the Newspapers were last replaced when they were alive, Pa and Ma died, cheese crust pizza declared "delicious," and war or something.
- 1997 - The Time Traveler's Pig - There was a costume store in the area the Mystery Shack is located. Maybe the Mystery Shack had a costume store as part of it? (Like how the Wax Museum was part of it.)
- 2002 - Headhunters - The wax figures have "been waiting 10 years to get revenge on [Stan] for locking us up."
- 5pm - Tourist Trapped - Mabel goes on her final date with "Norman" where he reveals himself to be a bunch of gnomes. Ugh.
- Saturday, 8pm - Headhunters - Toby Determined makes out with Shandra Jimenez' cardboard cutout, Wax Sherlock Holmes beheads Wax Stan Pines, and Manly Dan "punches the clock".
- 6:30pm - The Inconveniencing - Quitting time for Wendy.
- 12pm - The Time Traveler's Pig - Dunk Tank opens at the Mystery Fair.
- 6pm (approx.) - Mabel and Dipper travel back in time for the first time.
- Friday, June 22 - Summerween - Summerween Day
- 207̃012 - The Time Traveler's Pig - Year Blendin Blandin is from.
If I missed anything, tell me about it!