Day of week - simondotm/stardot-wiki GitHub Wiki
\ Based on code at http://6502.org/source/misc/dow.htm by Paul Guertin.
\ This routine works for any date from 1900-03-01 to 2155-12-31.
\ No range checking is done, so validate input before calling.
\
\ It uses the formula
\ Weekday = (day + offset[month] + year + year/4 + fudge) mod 7
\ where the value of fudge depends on the century.
\
\ On entry, A=day, 1..31
\ X=month, 1..12
\ Y=year-1900, 0..255
\ On exit, A=day of week 1..7 for Sun..Sat
.DayOfWeek
CPX #3:BCS dow_march :\ Year starts in March to bypass leap year problem
DEY :\ If Jan or Feb, decrement year
.dow_march
EOR #&7F :\ Invert A so carry works right
CPY #200 :\ Carry will be 1 if 22nd century
ADC dow_months-1,X :\ A is now day+month offset
STA dow_tmp
TYA:JSR dow_mod7 :\ Get the year MOD 7
SBC dow_tmp:STA dow_tmp :\ Combine with day+month
TYA:LSR A:LSR A :\ Get the year DIV 4
CLC:ADC dow_tmp :\ Add it to y+m+d and fall through to MOD 7
.dow_mod7
ADC #7:BCC dow_mod7 :\ Reduce A to A MOD 7
ADC #0:RTS :\ Increment to standard 1..7 range
.dow_months
EQUB 1:EQUB 5:EQUB 6:EQUB 3 :\ Month offsets
EQUB 1:EQUB 5:EQUB 3:EQUB 0
EQUB 4:EQUB 2:EQUB 6:EQUB 4
.dow_tmp
EQUB 0 :\ Temporary storage
You can test this with:
FOR Y%=1 TO 255
FOR X%=1 TO 12
FOR A%=1 TO 31
PRINT A%;"/";X%;"/";1900+Y%;" ";
PRINT MID$("SunMonTueWedThuFriSat",((USRDayOfWeek)AND&FF)*3-2,3)
NEXT A%:NEXT X%:NEXT Y%
I must say that this is an impressive bit of code!