Notes: GH and FR both happened in the past, relative to Modern Earth:
"The Savant-Sage and Pluffet Smedger the Elder are part of the past. The World of Greyhawk Fantasy Setting is yours now, to do with as you wish. You can mold new states from old, or inflame ancient rivalries into open warfare, as you tailor the world to suit the needs of your players. The time has come for new legends to be created, new battles to be fought, new songs to be sang. It is your world - welcome to it!"
-Steve Winter & Allan Hammack
For 5E, I'd like to see a coherentization of the timelines of all the worlds. Here's what WotC would need to do to get it right:
First, gather all of the canonical cross-over dates from Spelljammer and Planescape which have already been identified by aficionados (and tertiarily, from Ravenloft, though the Mists seem to skew time unpredictably, moreso than space-travel and planewalking).
Next, discern and pick the most coherent, most fitting set of cross-over correspondences. Explain away the remainder of the cross-over correspondences which don't fit - consider them to be alternate timelines, or blips wherein there was unwitting timetravel involved.
Next, be sure to take into account that some of the worlds have years which are shorter (or longer?) than 365 days! Yes, sit down and do the math. Then calculate the dates, and combine the canonical events from all D&D worlds onto a single timeline, showing the date and time from all the known calendars from all D&D worlds.
Next, find out the "most advanced current date" for all of the key D&D worlds. For example, the canonical Ravenloft timeline has advanced to 758 BC, and Mystara was canonically left at 1016 AC in the last AD&D2E product. So find "the most advanced current date".
The "most advanced current year" of the various worlds (as of April 2018)
FR: DR 1491, Princes of the Apocalypse, though five Adventurers League seasons have occurred since then. Ed Greenwood responded to my question on fb: "My novel DEATH MASKS takes place in 1491 DR, the Year Of The Scarlet Witch. I don't recall any events beyond that year in either Erin's or Bob's last-so-far Realms novels...so that's where we officially stand. Beyond that, NDAs seal my lips, I'm afraid. ;}" But there's a Mirt story set in DR 1492. http://www.dmsguild.com/product/199519/ALL-THINGS-THROUGH-THE-BRIGHT-FLAMES--The-First-Tale-of-Mirt-In-the-Time-After-The-Sundering
GH: CY 598, last year of Living Greyhawk modules.
Dragonlance: 430 AC, where in Solamnia is now a combined empire under the rule of James Markham.
Mystara: 1016 AC, Karameikos: Kingdom of Adventure boxed set. (Not counting "future timelines" such as Master of the Desert Nomads in 1200 AC):
Ravenloft: 758 BC (the classic timeline) and 735 BC (the Curse of Strahd timeline, which is a different version of Ravenloft): http://fraternityofshadows.com/wiki/Timeline. Fraternity of Shadows says: "758 was the year of the "latest" (in time) officially published products (Gaz V & Dark Tales & Distubing Legends) in the last year that Arthaus/WW published Ravenloft. (2004) Many fans have extended the timeline on a year-for-year basis, so it's now unofficially 772 (in 2018). Curse of Strahd exists in a separate timeline than the other Ravenloft products. The book says it takes place in 735, but that's the same year the original "Black Box" setting took place. But the original I6 adventure was placed in 528 by many later products, and Ireena Kolyana appears in both, at apparently the same age. So it's not really possible to reconcile them. CoS took characters from different points in the timeline and put them together in 735 as if they'd always been there." (See also the possibility of the events occurring in a dream sequence in 528 BC: https://www.reddit.com/r/CurseofStrahd/comments/88xy87/when_in_the_timeline_should_curse_of_strahd/.) The Barovian Calendar is widely used. There's also a Forfarian Calendar: 750 BC = 2137 Forfarian Calendar. Patriarch's Calendar is the calendar used in Vorostokov
Eberron: Wiki says present year is 998 YK (Year of the Kingdom). Rising From The Last War (5e) is set (by default) in 998 YK, four years after the Day of Mourning and two years after the Treaty of Thronehold brought the Last War to an end.
Dark Sun: What year did 3E DS advance to when it went 300 years into the future? In 4E it went back to Free Year 1.
Birthright: website says "current date" 551 MR.
Next, do either one of two things:
A) Advance all the timelines (from "the most advanced current date") to match the current Forgotten Realms year. Which would mean that the worlds which were left behind in 2E (e,g, Mystara and Birthright) would be jumping forward over a hundred years. Give a brief synopsis of how the cosmological changes from 2E to 3E to 4E to 5E happened in each world (e.g. from the perspective of the other worlds, surely their own gods played a role alongside Mystra and Shar), and date exactly what year those happened, based on their timing in the Forgotten Realms.
OR
B) Don't jump the timelines forward, but instead just take "the most advanced current date" of all the worlds, and just say that those are all now the same year. Like: "That just happened last year." So time may have flowed at a different rate in different worlds in the past. But not now.
In the case of A, all the worlds experienced something like the Spellplague and the Second Sundering (though they called the events by other names). These catastrophes happened at the same time as the FR catastrophes happened. The wider emergence of Sorcerers (in 3E) and Warlocks, Dragonborn, and Tieflings (in 4E and 5E) would be roughly synchronized with their emergence in Toril. According to option A, a straightforward interpretation of canon is that each D&D world experienced something like the Time of Troubles (1E > 2E), Die Vecna Die!/Apocalypse Stone (2E > 3E), Spellplague (3E > 4E), and the Second Sundering (4E > 5E), but called the events by different names. All those worlds (Abeir-Toril, Oerth, Krynn, Mystara, Aebrynis (Birthright), Athas (Dark Sun)) were an integral part of the 2E Great Wheel cosmology. And Die Vecna Die! explicitly said that the the whole multiversal framework was changing. Presumably the other "edition-change-based cosmological transformative events" affected all of the worlds...we just didn't hear about it, since WotC was no longer publishing books about them.
Greyhawk had its own Time of Troubles, detailed in the Fate of Istus, which provided in-story reasons for changing from 1E to 2E.
Krynn had the Chaos War (1e Krynn to SAGA Krynn) and War of Souls (SAGA Krynn to 3e Krynn)
And Mystara was implicitly affected by the 3.5E changes to the Plane of Nightmare/Plane of Dreams/Far Realm described in Dragon #327 "Winning Races: Diaboli--Bringing Diaboli into 3.5E"
So, yeah, Sigil and Oerth (and all the other D&D worlds) were affected in a big way by the cosmological changes which occurred at the same time as Toril's Spellplague and Second Sundering, but they wouldn't call the events by those names.
In the case of B, for the other worlds (besides Toril) there'd be no "Spellplague" or "Second Sundering" marking the switch to new editions. (Though Dragonlance had its own events for some of these edition switches.) In option B, these worlds would just jump from 2E or 3E straight to 5E overnight, without batting an eye. Dragonborn and Tieflings were always there (just off-screen), but are just now emerging in a more visible way this year. According to Option B, the canonical timeline of all the "retired" worlds stopped when the last product was published. And now they just appear through a 5E lense, just one year later. But now its all lined up, and its all 5E. No catastrophes.
Both options could be justified according to canon.
Actually...DO BOTH. In the upcoming setting books, I'd like WotC to recognize that there are two two different official D&D Multiverses, with two different timelines:
Classic Timeline: The story of the classic D&D worlds, which unfolded exactly as presented in the old product lines. This would be like the "Legends Continuity" of Star Wars. The Classic Timeline of all the worlds would be tied together using existing Spelljammer/Planescape cross-over dates, to make a unified "Classic Multiverse". Localized versions of the events of Die Vecna Die, the Spellplague, and Second Sundering happened "off-screen" in the future of all the non-FR worlds.
5E "Reboot" Timeline. As seen in Curse of Strahd. Also like the new "Story Group Continuity" of Star Wars or the Abramsverse of Star Trek. Reset the date to most iconic era of each world (e.g. Mystara 1000 AC) - and equate these to the current year of the 5E Forgotten Realms to make a unified "Reboot Multiverse". This timeline may mix and match most iconic characters from different eras of the Classic timeline. In the Reboot Multiverse, the mulitiversal catastrophes of Die Vecna Die, the Spellplague, and Second Sundering didn't necessarily happen in the other worlds.
Even though the Reboot Multiverse would be the one which is currently supported, the Classic Multiverse would be acknowledged as continuing to exist as a parallel D&D Multiverse. Forgotten Realms would be the same in both Multiverses, since it's the only world which maintained continuity and advanced its timeline throughout all editions.
There are also some timeline fragments which don't fit into either timeline (such as the Great War future timeline of Mystara, which was later retconned), and which would be like the Infinities (N-Canon) stories of Star Wars, which are neither Legends Universe or Story Group Universe.
Also, each DM's own campaign is a distinct Multiverse as well.
(Be sure to take into account that the various "rules editions" are canonically different Realities (entirely different "rules-based" parallel Multiverses) which have always existed, and continue to exist. Which is proved by how the War of the Lance has been portrayed using 1E rules, 2E rules, and 3E rules. And how the Arcane Age of Forgotten Realms used 2E rules (though there were suggestions for modelling pre-Time of Troubles characters and magic using 2E.) We'd just be seeing a different Reality from now on, the 5th Edition Reality.)
Also include Earth. Namely, all of the Earth-based TRPG settings (Masque of the Red Death, d20 Modern campaign models, Gamma World, Star Frontiers, the Earth cross-overs with Greyhawk (Murlynd), though Pluffet Smedger in original Greyhawk book says Oerth long ago disappeared(?), FR (Mulhorand and Unther), Mystara (Averoigne, Laterre, Immortal-level adventure which visits Chicago), Dragonlance (Wizards Three visit Lake Geneva)). Each "campaign model" is a different Tangent though...an alternate timeline of Earth. Line up the timelines of the various Earth Tangents.
But once the timelines are all lined up like this...keep advancing them all at the same rate. As the Forgotten Realms timeline moves forward via novels and Adventurers League, all of the other worlds move forward too, at the same pace. Perhaps publish a little "almanac" update for the worlds each year. Make a simple online digital "Perpetual Calendar of the D&D Multiverse" which calculates how the days line up from now on.
But the "iconic eras" can still be visited and supported. (Such as how the Curse of Strahd was set in 735 BC instead of the more current date of 758 BC. Or how the vast majority of Mystara products are set in 1000 AC, or how Free Year 1 is an "iconic era" in Dark Sun even though 3E skipped forward 300 years.) Just have to use time-travel. When Planescape and Spelljammer are brought back for 5E, be sure to include Chronomancer rules, so that all of these adventure sources can be mined by time-hopping adventures (along with Mystara's Age of Blackmoor, the legendary eras in Dragonlance's River of Time, the Arcane Age of Forgotten Realms, Greyhawk 2000, and so forth.)
Timeline correlations (toward a Grand History of the Multiverse):
Temporal Chronology of the Primes: https://web.archive.org/web/20110711...chronology.htm
Spelljammer Timeline: http://www.spelljammer.org/essays/hi..._timeline.html
Unified Multiversal Timeline: http://planejammerchronicles.com/about-2/
Timaresh's Timeline: http://www.rilmani.org/timaresh/Timeline
Timeline Converter: https://web.archive.org/web/20040720...nd/chrono.html
Also relevant is TSR's "Chronomancy and the Multiverse" (RTF download) which notionally includes (though without dates) many worlds within the D&D Multiverse, including such obscure worlds as Historical Reference Campaign & Masque of the Red Death (D&D Earth), Gamma World, and Council of Wyrms.
http://www.enworld.org/forum/showthread.php?358019-Grand-History-of-the-Multiverse#ixzz3AxZFuIWd
FR recent events (5E adventures): http://www.forum.candlekeep.com/topic.asp?TOPIC_ID=19630
The thread I started at ENWorld: http://www.enworld.org/forum/showthread.php?631624-Coherentizing-the-Timelines-of-all-the-D-amp-D-Worlds-in-5E-(and-Ed-Greenwood-s-current-year)&p=7391554&viewfull=1#post7391554
Mystara is one key world which hasn't been clearly synchronized with the other timelines.
Here are the known crossovers between Mystara and the other worlds: http://pandius.com/mystchar.html
(I'd add to this list the world-gates which were mentioned in the AD&D conversion guide in the back of some of the Gazetteers and connect Mystara with Greyhawk and Forgotten Realms. And also the world-gates from the Book of Marvelous Magic, which connect Mystara to Greyhawk, Dawn Patrol, Boot Hill, Gangbusters, and other early settings.)
Here's one enthusiast's attempt: http://www.thepiazza.org.uk/bb/viewt...133501#p133501
And discussion of details here: http://www.thepiazza.org.uk/bb/viewtopic.php?f=3&t=3223 The discussion includes future events, such as correlations with the Greyhawk 2000 "D&D Modern" setting.
For reference, here's the existing Mystara timeline: http://pandius.com/timeline.html
Mystara cross-overs:
Mystara was, for most of its existence, not part of the Advanced Dungeons & Dragons multiverse as originally described in the Player’s Handbook and Dungeon Master’s Guide. It was the setting instead for the Dungeon & Dragons game, and grew from the module X1 Isle of Dread, which sketched out the world, and was included in all of the 1980 Dungeons & Dragons: Expert Sets. In 1994 TSR converted Mystara to the Advanced Dungeons & Dragons system and subsequently made some minor attempts to add it to the multiverse setting.
In Domains of Dread the Dark Lord Meredoth is said to come from the Alphatian province of Norwold, arriving in 635 of the Barovian calander. (p79) But Norwold was not colonized by Alphatia until 919 AC according to Dawn of the Emperors (Bk III, p7). This would put Mystara wildly out of sync with the other campaign settings, and can be discounted as an example of Ravenloft’s famous mists transferring their victims through time as well as drawing them across planes.
There are some Planescape/Mystaran cross-overs. The Planewalker's Handbook, set in 128 Ha (p53), describes the Alphatian Empire as “fallen” (p33). We know the Alphatian Empire disappeared from the surface of Mystara in 1009 AC as depicted in Wrath of the Immortals (p??). Additionally, Uncaged: Faces of Sigil, set in 128 Ha, describes a Shadow Elf, Farrow, as an inhabitant of Sigil (p32). Farrow is spying in the Great School of Glantri when a Glantrian mage forcibly sends him to Sigil through a gate. It is unclear how long Farrow has been in Sigil, but certainly a few years. This provides a broad link between current time in Planescape and Mystara.
There is an equally vague, yet direct, link between Mystara and Spelljammer. The introductory Advanced Dungeons & Dragons boxed set First Quest includes a Spelljammer adventure in which the antagonists are heading for the Rock of Bral (AdvBk, pp. 45-54). Karameikos: Kingdom of Adventure is contemporary with First Quest (p??) and is set in 1012 AC (this aligns with Poor Wizard’s Almanac III, also set in 1012 AC and published in 1994 like Karameikos: Kingdom of Adventure and First Quest. There is no indication that the Rock as mentioned is not contemporary with its description in SJR5 Rock of Bral, which is set in 5049 OC. This fits with the Planescape links above, as the fall of Alphatia in 1009 AC would coincide with 124 Ha.
So we have 1012 AC linked to 5049 OC, which is 1367 DR. That would make 1361 DR equal to 1006 AC.
***
Other notes:
"Blood War" mentioned in Dark Knight of Karameikos. And Harmonium paladin wears red steel.
Urt (of Mentzer's BECMI boxed sets) is really a different world than Mystara (of Bruce Heard's Wrath of the Immortals cosmology). Urt (at least in one Tangent) is the Jurassic past of Earth, and so is the same size as Earth, and is a living creature called a Megalith. Whereas Mystara is said to be smaller than Earth, and is hollow, and exists in a different Dimension (Earth exists in the Dimension of Myth, while Mystara exists in the Multiverse Dimension).
The three Birthright calendars:
There are three principal calendars in common use in Cerilia today: Masetian Arrival (Ma), used by the Khinasi; Haelyn's Count (HC), favored by the astronomers and scholars of Anuire, and commonly used elsewhere in Cerilia; and Michaeline Reckoning (MR), in common use in Anuire and Rjurik. Masetian Arrival starts its count of years from the first arrival of humans in Cerilia, at the end of the Flight from Shadow. Haelyn's Count begins with the ascension of Haelyn as a god, at the Battle of Mount Deismaar. Michaeline Reckoning is dated from the death of Michael Roele.