program ZellerPascal;
{$mode objfpc}{$H+}
// Zeller's Congruence implementation
// Returns the day of the week for a given date.
function dayOfWeek(d, m, y: Integer): String;
// Zeller's output mapping:
// 0 = Saturday, 1 = Sunday, 2 = Monday, ... 6 = Friday
const
names: array[0..6] of String = (
'Saturday', 'Sunday', 'Monday', 'Tuesday',
'Wednesday', 'Thursday', 'Friday'
);
var
K, J: Integer;
// Zeller's formula (clearer step-by-step version):
term1, term2, term3, term4, term5, term6: Integer;
h: Integer;
begin
// In Zeller's formula, January and February are counted
// as months 13 and 14 of the previous year.
if m < 3 then
begin
m := m + 12; // Convert Jan → 13, Feb → 14
y := y - 1; // Move to previous year
end;
K := y mod 100; // Year of the century (last two digits)
J := y div 100; // Zero-based century (e.g., 2024 → 20)
// Zeller's formula (clearer step-by-step version):
term1 := d; // day of month
term2 := (13 * (m + 1)) div 5; // month adjustment
term3 := K; // year of century
term4 := K div 4; // leap years in century
term5 := J div 4; // leap centuries
term6 := 5 * J; // century correction
// Combine all terms and take modulo 7
h := (term1 + term2 + term3 + term4 + term5 + term6) mod 7;
// Return the corresponding weekday name
Result := names[h];
end;
var
d, m, y: Integer;
begin
d := 30;
m := 5;
y := 2024;
WriteLn(dayOfWeek(d, m, y));
end.
(*
run:
Thursday
*)
