Diary sexp and comparing dates, agenda

[David Rogers]

Hello all

I'm using the Org agenda to show when certain church occasions 
will happen. Mostly I've got them working correctly, after 
"stealing" the method used in holidays.el for finding the date of 
Easter, along with someone else's function that then uses that to 
find the difference between Easter and today. (I'm pretty sure 
that whole idea was on emacswiki, and I think Paul Sexton put it 
together.)

So now I have a much smaller problem to solve, but I don't 
understand how to get it to work; I suspect it has to do with how 
dates get formatted within different functions. Using what I 
already have, I can do this:

* Example 1
<%%(= 245 (mf-days-from-easter))>

because "mf-days-from-easter" is defined in my init file, along 
with the definition of Easter itself. This does what it looks like 
it should do; this year, Example 1 is shown on December 1.

And I can do

* Example 2
<%%(and (diary-float 1 0 5 7) (<= (mf-days-from-easter) -56))>

to say "five Sundays after January 6th, but only if Easter is 
still 8 weeks away or more".

But Example 1 isn't quite finished, because that date is too close 
to Christmas. I can easily show a diary sexp defining when "too 
close to Christmas" is:

<%%(diary-float 12 0 -4 24)>

So I want to combine these ideas, to say "Show Example 1 in the 
agenda 245 days after Easter each year, but only if it's earlier 
than the fourth Sunday before Christmas".

I could do a long string of (and (not this day, not this day, etc 
etc [insert long list of days] . . . , but that seems like a last 
resort.

Everything in this question fits into "the current year according 
to the agenda view" - there's no need to consider dates that cross 
a year boundary.

So: Is there a fairly simple way to define a (mf-days-from-advent) 
that will do a similar job to what (mf-days-from-easter) is 
already doing (i.e. it works when used in a diary sexp)?


Here are the relevant definitions I've been using:


   (defun mf-easter (displayed-year)
       (let* ((century (1+ (/ displayed-year 100)))
              (shifted-epact               ; age of moon for 
              April 5...
               (% (+ 14 (* 11 (% displayed-year 19)) ; ...by 
               Nicaean rule
                     (-     ; ...corrected for the Gregorian 
                     century rule
                      (/ (* 3 century) 4))
                     (/       ; ...corrected for Metonic cycle 
                     inaccuracy
                      (+ 5 (* 8 century)) 25)
                     (* 30 century))       ; keeps value positive
                  30))
              (adjusted-epact              ; adjust for 29.5 day 
              month
               (if (or (zerop shifted-epact)
                       (and (= shifted-epact 1) (< 10 (% 
                       displayed-year 19))))
                   (1+ shifted-epact)
                 shifted-epact))
              (paschal-moon ; day after the full moon on or after 
              March 21
               (- (calendar-absolute-from-gregorian (list 4 19 
               displayed-year))
                  adjusted-epact)))
              (calendar-dayname-on-or-before 0 (+ paschal-moon 
              7))))



   (defun mf-days-from-easter ()
     "When used in a diary sexp, this function will calculate how 
     many days
are between the current date (DATE) and Easter Sunday."
     (- (calendar-absolute-from-gregorian date)
        (mf-easter (calendar-extract-year date))))


--
David