Cut and paste from a mini-essay I wrote up for the Timeline project ...
Early Terran space exploration
Dates differ a little between different GDW sources, between GDW sources and GURPS sources, and depending on whether the date is referenced in the Imperial or the Solomani dating systems. The translation factor between the Imperial and Solomani dating systems also frustratingly) differs between sources, and sometimes within a single source. For most sources (CT:AM6, MegaTraveller GDW and DGP works, TNE), Imperial date + 4518 = Solomani date (A.D.) This means the widely quoted baseline of “-4520 = 1 A.D.” should instead be -4517 = 1 A.D.
Exceptions to this +4518 translation are:
• In C:A2 (Research Station Gamma), the first official equivalency of dates was given. Imperial + 4521 = Solomani (A.D.) This equivalency also applies to the Solomani dating chronology in C:A5 (Trillion Credit Squadron).
• C:AM6 (Solomani) gives a discussion of the calendar on p.30 with equivalent dates: Imperial + 4523/22/21/20 = Solomani (A.D.); the years do not have a one-to-one correspondence, with +4523 in the 20th century, and +4520 in the 56th century. However, dates of major events before the Interstellar Wars are also given in both Solomani dates (pp.4-5) and in Imperial dates (p.12): Imperial + 4518 = Solomani (A.D.) for the development of the jump drive, and for contact with the Vilani; and Imperial + 4526 = Solomani (A.D.) for the start of the first Interstellar War.
• G:T 1st edition gave a comparative chronology that matched most sources (ie. +4518 translation factor), complete with the -4520 = 1 A.D. discrepancy. Published errata explicitly changed this to Imperial + 4521 = Solomani (A.D.)
• In the Traveller Integrated Timeline, real-life events of the late 20th century sourced from the Traveller Digest issues use Imperial + 4519/4520 = Solomani (A.D.)
• G:IW uses Imperial + 4522/4523 = Solomani (A.D.), except for events around the development of jump drive and contacting the Vilani, which use Imperial + 4519 = Solomani (A.D.)
Most problems come from the above two translations (assuming a fudge of +/- 1 year due to the calendar years not necessarily beginning on the same day). I have used the following interpretations for dates in the different publications.
1. The Imperial date as described in “most sources”, as described above, is the gold standard.
2. For “most sources”, using a translation of Imperial date + 4518 = Solomani date (A.D.) will provide the Solomani date in the publication. This is the “gold standard Solomani date”.
3. Ignore the equivalency of dates given in C:A2, C:AM6, and the G:T errata. The +4521 translation, or its more complicated variant in C:AM6 is no longer gold standard.
4. The Islands Subsectors chronology in C:A5 is no longer correct. Fortunately, it is corrected by the chronology given in N:RS (Regency Sourcebook) ... except the date given for the ESA mission is 2050 A.D. = -2452, a translation of +4502. A translation of +4518, like the remainder of the timeline, gives either an ESA mission date of -2468/2050 A.D., or a mission date of -2452/2066 A.D. The former is more consistent with previous publications, the latter makes more narrative sense (the long-range missions are consequently a few years after the initial mission to Alpha Centauri).
5. The Timeline in G:T 1st edition as originally published was (mostly) correct. -4517 = 1 A.D. should be in the timelines in both M:IE and G:T.
6. G:ROF is a dog's breakfast. Dates are given in the Imperial system, but conflict with prior sources for events up until the Interstellar Wars. It seems as though “gold standard Imperial dates” for events were converted into Solomani dates by adding 4518 (or by using the Solomani dates given in “most sources”) then converted back into imperial dates by subtracting 4521. The dates after -2408 (the start of the First Interstellar War) do match prior sources. For published Imperial dates before -2408, add 4521 to get the “gold standard Solomani
date”, or add 3 to get the “gold standard Imperial dates”*. For -2408 and after, add 4518 to get the “gold standard Solomani date; no correction is needed for the printed Imperial dates.
* The one exception I made to this was to keep the launch date for the generation ship to Alpha Centauri at -2468, to prevent it departing after the generation ships to the Islands subsectors.
7. For GURPS Interstellar Wars, subtract 4 or 5 years from any precise Solomani date printed to get the Solomani date that concords with other sources. The only exception is that real world events of the 20th century and before keep their real world date, and events around the development of jump drive and contacting the Vilani should have 1 year subtracted to get their “gold standard Solomani date”.