> for example, one can easily see that this algorithm has a Y2K1
> problem

It's well past important now, but what problem did you see ?

I've got this rough draft, seems to be working (so far) and tried a
few 2000 dates

For example 1st Jan comes back as day 1, 31st Dec comes back
as day 366, 29th Feb as day 60

Anyway, it appears I have got the calculation correct now. Because
of the limited range of results there some assumptions and shortcuts
that can be made, which will help to streamline it. I decided on an
addition method for the /9 because the * 0.111111 I got from PIClist
was drifting away from the correct result as the dividend got larger,
and speed isn't an issue

Thanks for the help. Gives me confidence in the rest of the algorithm

Now for the triggy bit ........... ;-)

Joe

;1. first calculate the day of the year
;
; N1 =3D floor(275 * month / 9)
; N2 =3D floor((month + 9) / 12)
; N3 =3D (1 + floor((year - 4 * floor(year / 4) + 2) / 3))
; N =3D N1 - (N2 * N3) + day - 30

date     movlw   .1
         movwf   day
         movlw   .1
         movwf   month
         movlw   high(.2000)
         movwf   year_hi
         movlw   low(.2000)
         movwf   year_lo

;=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
;  N1 =3D (275 * month)/9
;
;  month * 275
;=3D month * 256 + (2 * (month * 8)) + 3 * month

         movff   month,temp1
         clrf    temp2
         movff   month,temp3     ;month * 256
         clrc
         rlncf   temp1           ;month * 8
         rlncf   temp1
         rlncf   temp1

add_m8   clrc                    ;add month * 8
         movfw   temp1
         addwf   temp2           ;once
         addwf   temp2           ;twice
         movfw   month
         addwf   temp2           ;add 3 * month
         addwf   temp2
         addwf   temp2

         call    div9

         movff   temp0,n1_hi     ;N1
         movff   temp1,n1_lo

;=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
; N2 =3D floor((month + 9) / 12)

         movlw   .1              ;pre-set N2
         movwf   n2
         movlw   .3              ;if month < April, n2 =3D 0
         subwf   month,w
         bnn     do_n3
         clrf    n2

;=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
; N3 =3D (1 + floor((year - 4 * floor(year / 4) + 2) / 3))

do_n3    movff   year_lo,temp1   ;copy
         movff   year_hi,temp0

         clrc                    ;floor(year/4)
         rrcf    temp0
         rrcf    temp1
         clrc
         rrcf    temp0
         rrcf    temp1

         clrc
         rlcf    temp1           ;4 * floor(year/4)
         rlcf    temp0
         clrc
         rlcf    temp1
         rlcf    temp0

         movff   year_lo,temp3   ;copy
         movff   year_hi,temp2

         movfw   temp1           ;subtract temp0:temp1
         subwf   temp3           ;from temp2:temp3
         movfw   temp0
         skpc
         incfsz  temp0,w
         subwf   temp2           ;result in temp2:temp3

         movlw   .2              ;pre-set N3
         movwf   n3

         movf    temp3           ;if result =3D 0 then N3 will be 1
         skpnz
         decf    n3              ;else N3 will be 2

; N =3D N1 - (N2 * N3) + day - 30

         clrf    temp0
         clrf    temp1

         movf    n2
         bz      n1_sub          ;if N2 =3D 0, skip (N2 * N3)
         movfw   n3              ;N2 * N3
         addwf   temp1

n1_sub   movfw   temp1           ;N1 - (N2 * N3)
         subwf   n1_lo
         movfw   temp0
         skpc
         incfsz  temp0,w
         subwf   n1_hi

         movfw   day             ;+ day
         clrc
         addwf   n1_lo
         skpnc
         incf    n1_hi

         movlw   .30             ;- 30
         subwf   n1_lo
         movlw   .0
         skpc
         movlw   .1
         subwf   n1_hi

         nop

;=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D
;        Integer /9
;=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

div9     comf    temp2           ;complement for count up
         comf    temp3
         incf    temp2

         clrf    temp0           ;addition counter
         clrf    temp1

add9     movlw   .9
         addwf   temp2           ;add 9 to low byte
         skpnc
         incf    temp3           ;if *275 low byte > 255
         incf    temp1           ;increment counter low
         movf    temp1
         skpnz                   ;if <> 0
         incf    temp0           ;else increment counter high
test0    movf    temp3
         bnz     add9            ;if *275 high byte <> 0
         movf    temp2
         skpz                    ;if month =3D 9
         decf    temp1           ;else decrement count
         return
--=20
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