Add Zeller's Congruence Algorithm in Math

Maths/ZellersCongruenceAlgorithm.js

@@ -0,0 +1,26 @@
+// 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]
+}

Maths/test/ZellersCongruenceAlgorithm.test.js

@@ -0,0 +1,17 @@
+import { zellersCongruenceAlgorithm } from '../ZellersCongruenceAlgorithm'
+
+function testZeller (day, month, year, expected) {
+  test('Testing on ' + day + '/' + month + '/' + year, () => {
+    expect(zellersCongruenceAlgorithm(day, month, year)).toBe(expected)
+  })
+}
+
+test('Testing on this/should/throw', () => {
+  expect(() => {
+    zellersCongruenceAlgorithm('this', 'should', 'error')
+  }).toThrowError(new TypeError('Arguments are not all numbers.'))
+})
+testZeller(25, 1, 2013, 'Friday')
+testZeller(26, 1, 2013, 'Saturday')
+testZeller(16, 4, 2022, 'Saturday')
+testZeller(25, 4, 2022, 'Monday')