jd is the Julian Day Number to
convert. sg specifies the Day of Calendar Reform.
Returns the corresponding Commercial Date as
[commercial_year, week_of_year, day_of_week]
# File lib/date.rb, line 459
def self.jd_to_commercial(jd, sg=GREGORIAN)
ns = fix_style(jd, sg)
a = jd_to_civil(jd - 3, ns)[0]
y = if jd >= commercial_to_jd(a + 1, 1, 1, ns) then a + 1 else a end
w = 1 + ((jd - commercial_to_jd(y, 1, 1, ns)) / 7).floor
d = (jd + 1) % 7
d = 7 if d == 0
return y, w, d
end