[Remind-Fans] calendar blue moon

[xing at synapse.plus.com]

hi all. i have the following calculations to display the next calendar blue
moon:

# work out when the next 'calendar blue moon' is
# (second full moon in one calendar month)
# adapted from http://www.linuxjournal.com/article/3529
FSET _is_first_full(date) monnum(moondate(2, date)) == monnum(moondate(2, moondate(2, date)+1))
REM 1 SATISFY _is_first_full(trigdate())
IF trigvalid()
	SET blue moondate(2, moondate(2, trigdate())+1)
	REM [trigger(blue)] +100 MSG %"Blue moon%" is at [moontime(2, trigdate())] %a.
ENDIF

the calculation works well and it calculates the next calendar blue moon on
31 Dec 2008 in accordance with
http://www.obliquity.com/cgi-bin/bluemoon.cgi?year=2009

the first full moon in Dec 2009 is on the 2nd. when i display the reminders
after the 2nd of Dec using e.g.:
$ rem -q 5 Dec 2009
it calculates the next blue moon (31 Aug 2012) instead of displaying the 2009
one until it has passed.

any ideas on how i can display the 2009 blue moon until it has actually passed?

thanks in advance.

x