I've finally got it working. Seems like I've got the 'OnModActivated -> UpdateDatabase' thing wrong.
Of course, if you do that without ALSO changing the year:turn ratios in the GameSpeeds tables, your game will end at just past 4000 BC, because the game has been told "add A years per turn for the first B turns, then add C years per turn for the next D turns..." and so on down the line. So instead of ending at ~2000 AD, you'll end 6000 years earlier.
Altering THAT takes a lot more work, since there are four game speeds to modify, and adjusting those entries will also change what year a player starts in if they choose a late-era start. So it's not as simple as using an SQL command to double all of the entries in that table to make a 12000 year span instead of 6000, since you probably don't want your Industrial era beginning in 1500 AD.
I've made a custom game speed before trying to modifie the starting time. Basically I've raised the tech research to 600, but lowered building units and buildings to 150. Problem was, in my game (prince difficulty) I reached medieval age after ~250-275 rounds (year 1000-750 BC). I'm using this setup now starting at 10.000 BC:
600 (50 years) x 150 rounds = 7500 years = 2500 BC
300 (25 years) x 100 rounds = 2500 years = 0 (Birth of Christ)
120 (10 years) x 100 rounds = 1000 years = 1000 AC (Latest date of reaching medieval era)
60 (5 years) x 100 rounds = 500 years = 1500 AC (Beginning of the conquest of America)
24 (2 years) x 150 rounds = 300 = 1800 AC (Time of the European Revolutions)
12 (1 year) x 100 = 100 = 1900 AC (Start of the World Wars)
6 (0.5 year) x 150 = 75 = 1975 AC (My birth date)
3 (0.25 year) x 200 = 50 = 2025 AC (Time of the first maned Mars exploration, according to NASA)
1 (0.0833 year) x 450 = 37-38 = 2062-2063 AC (Time of the first steps of colonizing Mars, according to NASA)
I'm playing with the YNAEMP by Gedeon, made my own custom Mod Pack, which I'm planning to release and use this in conjunction with mods like R.E.D, Info addict and the Cultural Diffusion mod.
Timeline is working good so far. I'm reaching classical era ~ 4000-3000 BC, medieval era ~ 250-500 AC, renaissance era ~ 1200-1300 AC. I'm not far enough in the game to assess if the other eras are reached on time, but I have good hopes it does. If it doesn't, I'm probably going to manually modifie every single tech in order to lengthen their research.