bugs - What calendar is Mathematica using for dates in the distant past?

It appears that Mathematica treats all dates as proleptic Gregorian dates by default, a hypothesis that can be easily tested by using AbsoluteTime to compute the Julian Day:

jd[t_] := AbsoluteTime[t]/86400 + 2.4150205*^6

but, while this works for recent dates and for some older ones, it yields results that differ from the correct result for some older dates by exactly a day

jd[{2012, 11, 24, 12}] - 2456256
0
jd[{1100, 11, 24, 12, 0, 0}] - 2123154
0
jd[{-3000, 11, 24, 12, 0, 0}] - 625660

1

notably for the reference date, 12:00 Universal Time on January 1, 4713 BCE in the proleptic Julian calendar (-4713-11-24 in the proleptic Gregorian calendar):

jd[{-4713, 11, 24, 12, 0, 0}]
1

What calendar is Mathematica using for these older dates, if not the proleptic Gregorian calendar?

Here, perhaps is another clue:

DateList[{-4713, 11, 24, 12, 0, 0}]
{-4713, 11, 25, 12, 0, 0}

DateList[{-3000, 11, 24, 12, 0, 0}]
{-3000, 11, 25, 12, 0, 0}

I'm no calendar expert, but this "canonicalization" doesn't map to any calendar I'm familiar with. Is this a bug?

---

Update: Version 9, behaves differently:

jd[{2012, 11, 24, 12}] - 2456256
0.
jd[{1100, 11, 24, 12, 0, 0}] - 2123154
0.

jd[{-3000, 11, 24, 12, 0, 0}] - 625660
365.

jd[{-4713, 11, 24, 12, 0, 0}]
366

and

DateList[{-4713, 11, 24, 12, 0, 0}]
{-4713, 11, 24, 12, 0, 0.}
DateList[{-3000, 11, 24, 12, 0, 0}]

{-3000, 11, 24, 12, 0, 0.}