D&D canon has demonstrated numerous times that the various campaign settings are in fact, connected to each other. Powerful beings from Greyhawk, Forgotten Realms, and even Athas have found ways into other campaign settings in the D&D stable of worlds. (Even Earth is included in the list of alternate Primes visited, interestingly enough.)
Irrespective of the means used to cross from one campaign setting world to the other, are there any solid timeline references, correlations, or overlaps (specific dates or even a range of dates) where it can be demonstrated what year (or range of years) a cross-setting traveler would end up in when moving from any given setting to any other given setting?
For example, it has been demonstrated that both Elminster and Mordenkainen have traveled to each other's home Prime, as well as to Earth (apparently during the 70's – 80's). This provides a basic frame of reference, however they both lived rather long lives, and thus only provides a common range of dates that overlap in Oerth and Abeir-Toril; namely the date range of the span of their lives, or at least after they both reached a certain level of power.
I am open to timelines from any edition of Dungeons and Dragons, and all official campaign settings in the D&D stable (Spelljammer, Maztica, Mystara, Al-Qadim, Ravenloft, Planescape, Greyhawk, Forgotten Realms, Birthright, Athas, Dragonlance, Red Steel, Odyssey, Lankhmar, Conan, and so forth). Also, as the presence of our modern Earth is explicitly included in D&D canon, using Earth-based cross-references to multiple campaign settings in order to establish a linkage is also acceptable. I would prefer to avoid time travel, unless said reference manages to establish a firm date link.
Thank you. I'm sure this will not necessarily be easy to research.
Best Answer
Master Chronology
Based on the link provided by Mark Horsfall, and the information Paul Westermeyer has compiled and compared in the linked article, the following information has been derived:
Sources
The three primary campaign settings for D&D are Greyhawk (GH), Forgotten Realms (FR), and Dragonlance (DL). These three have more material published than the other settings, and include the majority of the cross-time references. (Which other settings are: Spelljammer (SJ), Maztica (MZ, part of FR), Al-Qadim (AQ, Also part of FR), Mystara (MY), Ravenloft (RL), Planescape (PS), Birthright (BR), Athas (AT), Red Steel (RS), Odyssey (OD), Lankhmar (LM), Eberron (EB) and Conan (CO).)
There are four additional primary sources for chronological cross-time points: Spelljammer, Planescape, Ravenloft, and Dragon Magazine (DM) articles written by Ed Greenwood himself.
Please note that there are some contradictions in sources, so precedence is given to authors with more published works and to sources based on Spelljammer (as it is specifically designed to cross-bridge the settings) as well as the Big 3 (FR, GH, and DL).
FR/DL cross-time link
Based on comparative information from SJ, FR, DL, RL, and PS the best cross-time linkage is:
The RL data contradicts itself, weakening it as a source, and the PS data fails to trump the SJ data.
GR/FR cross-time link
Based on data from FR/RL/GH and DM sources the best cross-time linkage is:
Note that the DM articles were written by Ed Greenwood about the wizards Elminster, Mordenkainen, and Dalamar meeting in his home on Earth to discuss the magical affairs of Faerun, Oerth, and Ansalon with reference to various famous wars and other events; and as such provide a strong primary source for cross-time linkages. As such, the dates of these articles can be used to provide an Earth-time linkage as well, should such be desired.
Other cross-time links
Linkages to other settings frequently fall back on the publication date of the earliest release of said setting, combined with in-setting references, or lack thereof, to determine dates. For detailed reasonings, check the link if available - and I have saved a copy in case the link goes down.
Calendars
After all is compared, the following calendars can be synced, with confidence:
This corresponds roughly to the 1990's on our Earth, for those interested.