ZellersCongruenceAlgorithm.js

// Zeller's Congruence Algorithm finds the day of the week from the Gregorian Date. Wikipedia: https://en.wikipedia.org/wiki/Zeller%27s_congruence
export const zellersCongruenceAlgorithm = (day, month, year) => {
 if (typeof day !== 'number' || typeof month !== 'number' || typeof year !== 'number') {
 throw new TypeError('Arguments are not all numbers.')
 }
 const q = day
 let m = month
 let y = year
 if (month < 3) {
 m += 12
 y -= 1
 }
 day =
 (q + Math.floor(26 * (m + 1) / 10) + (y % 100) + Math.floor((y % 100) / 4) + Math.floor(Math.floor(y / 100) / 4) + (5 * Math.floor(y / 100))) %
 7
 const days = [
 'Saturday',
 'Sunday',
 'Monday',
 'Tuesday',
 'Wednesday',
 'Thursday',
 'Friday'
 ]
 return days[day]
}