find_easter_april_20.ts

/**
 * Calculates the date of Easter Sunday for a given year using the Anonymous Gregorian algorithm.
 * See https://en.wikipedia.org/wiki/Computus#Anonymous_Gregorian_algorithm
 *
 * @param year The year for which to calculate Easter.
 * @returns A Date object representing Easter Sunday for that year.
 */
function getEaster(year: number): Date {
  if (year < 1583) {
      throw new Error("Algorithm is valid only for Gregorian calendar years (1583 onwards).");
  }

  const a = year % 19;
  const b = Math.floor(year / 100);
  const c = year % 100;
  const d = Math.floor(b / 4);
  const e = b % 4;
  const f = Math.floor((b + 8) / 25);
  const g = Math.floor((b - f + 1) / 3);
  const h = (19 * a + b - d - g + 15) % 30;
  const i = Math.floor(c / 4);
  const k = c % 4;
  const l = (32 + 2 * e + 2 * i - h - k) % 7;
  const m = Math.floor((a + 11 * h + 22 * l) / 451);

  // Month: (h + l - 7 * m + 114) / 31 --> 3 = March, 4 = April
  const month = Math.floor((h + l - 7 * m + 114) / 31);
  // Day: ((h + l - 7 * m + 114) % 31) + 1
  const day = ((h + l - 7 * m + 114) % 31) + 1;

  // JavaScript Date month is 0-indexed (0=Jan, 1=Feb, ..., 3=Apr)
  // The algorithm returns month 3 for March, 4 for April.
  return new Date(Date.UTC(year, month - 1, day));
}

const startYear = 1900;
const endYear = 2100;
// Target date: April 20th. Month is 3 (0-indexed), Day is 20.
const targetMonth = 3;
const targetDay = 20;

const resultYears: number[] = [];

console.log(
  `Searching for years between ${startYear} and ${endYear} where Easter Sunday is April 20th...`,
);

for (let year = startYear; year <= endYear; year++) {
  try {
    const easterDate = getEaster(year);
    // Use getUTCMonth() and getUTCDate() because we created the date with UTC values.
    if (
      easterDate.getUTCMonth() === targetMonth &&
      easterDate.getUTCDate() === targetDay
    ) {
      resultYears.push(year);
    }
  } catch (error) {
      // Although the range starts at 1900, the check inside getEaster handles pre-Gregorian years if the range were different.
      console.error(`Could not calculate Easter for year ${year}: ${error.message}`);
  }
}

if (resultYears.length > 0) {
  console.log("\nFound years where Easter Sunday falls on April 20th:");
  resultYears.forEach((year) => console.log(year));
} else {
  console.log(
    "\nNo years found in the specified range where Easter Sunday falls on April 20th.",
  );
}

1919
1924
1930
2003
2014
2025
2087
2098