I suppose that it doesn't exist but maybe i missed some obscure reference...
anyone knows some reference to common event that would help to synchronize timelines ?
Because the Poor Wizard's Almanacs came out with one real-world year equalling one Mystaran year, it's convenient to assume that, for example, AD 1994 was AC 1012 - that is, that the Almanacs took place at the time they were published in the real world. Planescape products advanced one year for every real-world year, too, so 130th year of Factol Hashkar's reign was 1998. That would mean AC 1012 was the same as the 126th year of Hashkar's reign in Planescape.
This method is nice because it means that everything published by TSR or WotC during the same year can be assumed to have happened simultaneously. Unfortunately, things get out of synch when a setting advances its timeline unevenly, as happened (for example) with Dragonlance skipping forward generations at a time, and Greyhawk hasn't moved forward as quickly as you would have expected (although it's moving at 1 year/year now). But it works okay with Mystara and Planescape, which both advanced 1 year/year.
Synching Mystara's calendar with other settings is also problematic due to the fact that a Mystaran year is only 336 days, while (for example) Oerth's calendar is 364 days. This problem can be avoided by assuming that time flows quicker on Mystara than it does on Oerth.
and Greyhawk hasn't moved forward as quickly as you would have expected (although it's moving at 1 year/year now).
Greyhawk also made a couple of jumps, as I recall, first with the Greyhawk Wars and From the Ashes sets (which took Greyhawk to 586 CY), and then with the Living Greyhawk Gazetteer, which took Greyhawk to 591 CY (I believe). Current time there is 596 CY, and with Living Greyhawk they seem to be advancing things at a 1 year/1 year rate, so that helps.
Synching Mystara's calendar with other settings is also problematic due to the fact that a Mystaran year is only 336 days, while (for example) Oerth's calendar is 364 days. This problem can be avoided by assuming that time flows quicker on Mystara than it does on Oerth.
I'm not sure I understand how you mean that solves the problem? If time flows quicker, than it makes it even more complicated doesn't it? Or did you mean that time flows quicker on Oerth than on Mystara?
anyone knows some reference to common event that would help to synchronize timelines ?
I suppose that it doesn't exist but maybe i missed some obscure reference...
Yes, we discussed this issue several times, most recently in the Italian Mystara message board.
This was the result
http://www.25edition.it/forum/topic.asp?whichpage=2&Forum_Title=Ambientazione+%5BMystara%5D&Topic_Title=Timeline+comparate+Mystara%2Daltri+mondi&CAT_ID=23&FORUM_ID=98&TOPIC_ID=28299
There were similar discussions here and on dragonsfoot as well, IIRC.
Since the linked page is mostly in Italian, I'll summarize here the main points:
1) The Planewalker's Handbook links the present of Mystara (post-Woti), Dark Sun (post-Githyanki invasion), and Planescape (pre-Faction Wars) to that of Krynn, placing them near the appearance of Chaos in that world.
2) On Hallowed Grounds links Krynn (Dragons of Summer Flame) with Forgotten Realms (12 years after the Times of Trouble) and the Birthright present.
These, and a few other elements reported in the linked page lead to the conclusions reported in the figure -- the brown row reports the most likely equivalents of 1010 AC in other settings.
Note: the work on the Italian MMB assumes that a day's length is the same everywhere. Since we don't have multiple certain anchors, this may not necessarily be the sole possible choice.
Greyhawk also made a couple of jumps, as I recall
Yes, but not as many as you'd expect if time had flowed there as quickly as it has in the real world.
The 1983 World of Greyhawk boxed set (I'll ignore the earlier Folio) took place in CY 576. The Living Greyhawk Gazetteer (CY 591) was published in 2000, 17 years later in real time but only 15 years later in game time. Since 2001 (also assumed to be CY 591) the Dungeon Magazine and Living Greyhawk timelines have been advancing at 1 year/1 year, so 2006 is 596.
Or did you mean that time flows quicker on Oerth than on Mystara?
Yeah, that's what I meant. So one of Mystara's shorter years passes in the same "time" it takes one of Oerth's longer years to pass.
It might be easier to just assume that Mystara has longer days.