First of all, here is the short and simple HP 17Bii Solver equation: (both the solver and program assume DD.MMYYYY).
Code: Select all
ADVDATE = DATE( L( XMAS : YEAR ÷ 1000000 + 25.12 ) : ( ( MOD( DDAYS( 1.0119 : G( XMAS ) : 1 ) : 7 ) + 22 ) x - 1 ) )
Code: Select all
LBL "ADVDATE"
1000000
/
25.12
+
STO 00
1.0119
X<>Y
DDAYS ;XROM 26,14
7
MOD
22
+
CHS
RCL 00
X<>Y
DATE+ ;XROM 26,13
END
Enter YEAR: 2021
Solve ADVDATE: 28,112021
Running on the DM41L:
2021
XEQ ADVDATE
28,112021
Of course, the program (and equation) only works for the Gregorian calendar. Monday Jan. 1, 1900 was chosen because I liked that date (any date works, as long as you adjust the offset (currently 22) in the equation.