Assumptions:
TNE (Core Book page 143) specifies that Imperial year 1200 is the Solomani year 5717 (I assume AD according to Gregorian calendar, the one used today in western world).
There are a number of different canonical Imperial dates with their equivalent Solomani dates. Unfortunately, they don't match up. There's nothing inherently wrong with any of them, except that if some of them are right, others can't be and vice versa. So when we were working on
Sword Worlds, Jon and I decided to pick one date as true and calculate everything else from that. It's hardly rocket science, but it is tricky. It's also the sort of thing that it's unlikely that "real" historians from the TU would get wrong, since presumably their computers are as capable of converting as ours are. Anyway, the date we (me and Jon and Constantine Thomas) chose was 292-950 Imperial (13 March 5471 Terran), the date Margaret chose for decreeing the reintegration of the Solomani Sphere into the Imperium, as mentioned on p. 11 of AM6.
If 292-950 corresponds to March 13 5471, 001-0 corresponds to Jan 11 4521. Sadly, on another page of AM6, it is specifically stated that 001-0 is 19 January 4521, not 11 january 4521 (Note that this is the very same book that has the date for Margaret II's edict). If I had remembered about this date back when I was working on SW, we would almost certainly have chosen 001-0 = 19 Jan 4521 over 292-950 = 13 March 5471. But I didn't, so the date in SW matches the second and not the first. That's really the only reason I suggest sticking to that, but I think it's a pretty good reason, since there's about a dozen dates in SW that relies on it. YMMV.
According to this calendar, to convert it to Imperial one, we must add one day every 4 years (lap year), skipping the years ended in 00 unless they are divisible by 400. As those 1200 years would give us a discordance of 291 (less than one full year) I assume the Imperial year 0 is 4517 AD.
The current rule is 'Every year divisible by four, unless it is also divisible by 100, unless it's also divisible by 400'. So 3600 will be a leap year, but 3700, 3800, and 3900 won't be. If this scheme is not changed, 4000 will be a leap year. However, there's a very small discrepancy that is not accounted for currently. Making years that are divisible by 4000 not-leap years would increase the accuracy a tiny bit. Some people think this modification will be introduced some day, others think it won't. Someone (I think it was Jon Zeigler, but I won't swear to it) once told me that Marc Miller had told him (JZ) that he (MM) thought the change would be introduced. Not the most impressive of evidence, but nowhere in canon does it say that it won't be, so I was free to chose either way. When I wrote my part of Sword Worlds, I established that it had, if fact happened).
It's a decision I regret, because accounting for that discrepancy is a pain, but what's done is done.
I also assume that when Cleon I instituted the Imperial calendar, to ease the transition, it began year 0 in what it would be new year day in 2517 AD.
Not in any of the canonical versions.
Going by the GT version, the rules for figuring out Imperial dates are as follows:
To convert from Imperial date to Revised Gregorian date:
1) Figure out the Julian day of the date in question by comparing with known Imperial dates.
2) If the Julian day is 3182089 or more, add one to the number.
3) Run the number through any on-line Julian day calculator.
To convert from Revised Gregorian date to Imperial date:
1) Run the date through any on-line Julian day calculator.
2) If the Julian day is 3182089 or more, subtract one from the number.
3) Figure out the Imperial date in question by comparing with known Imperial dates.
The reason for the addition/subtraction of one for dates past 288-522 PI (28 Feb 4000/Julian day 3182088) is that the (unrevised) Gregorian Calendar has 4000 AD as a leap year. A Julian day calculator thus believes that Julian day 3182089 is Feb 29, 4000, but in the Revised Gregorian Calendar it is actually Mar 1, 4000.
LIBRARY DATA:
Revised Gregorian Calendar: The calendar used by the Solomani Confederation and a large number of worlds settled from the Old Earth Union after -536 (3985 AD.), including the Sword Worlds. It is identical to the original Gregorian calendar that it replaced, except that -522 (4000 AD.) is not a leap year. According to this calendar, 001-1105 is equivalent to April 18, 5625 AD.
(Note that the entry in
GT:Sword Worlds has an error: The 'not' is missing.)
A few dates for comparison:
Code:
Imperial Solomani Julian Day Event
date
328-2510 PI Jan 1 2014 2456659
001-2509 PI Feb 8 2014 2456697
152-2353 PI Jan 1 2170 2513637 Start of default IW campaign.
288-522 PI Feb 28 4000 3182088
----- Feb 29 4000 -- Exists in the Gregorian Calendar
but not in the Revised Gregorian,
289-522 PI Mar 1 4000 3182089 Gregorian and Revised Gregorian
dates diverge.
001-0000 Jan 11 4521 3372331 Third Imperium established.
Imperial dating system begins.
292-950 Mar 13 5471 3719372 Solomani Autonomous Region
dissolved by Margaret II.
001-1105 Apr 18 5625 3775658 Date of default MgT campaigns.
001-1120 Apr 14 5640 3781133 Date of default GT campaigns.
Hans
