--<pre>
-- Julian Day (after 1582 Oct 15)

JD(y,m,d) = (JD_BCD(y-1,m+12) when m < 3,
             JD_BCD(y,m) otherwise)+d+1720994.5

JD_BCD(y1,m1) = JD_B(trunc(y1/100))+
                trunc(365.25*y1)+
                trunc(30.6001*(m1+1))

JD_B(a) = 2-a+trunc(a/4)