added DateDayDifference method in Conversions category

Conversions/DateDayDifference.js

@@ -0,0 +1,41 @@
+/*
+    DateDayDifference Method
+    ------------------------
+    DateDayDifference method calculates the number of days between two dates.
+
+    Algorithm & Explanation : https://ncalculators.com/time-date/date-difference-calculator.htm
+*/
+
+// Internal method for make calculations easier
+const isLeap = (year) => {
+  if (year % 400 === 0) return true
+  else if (year % 100 === 0) return false
+  else if (year % 4 === 0) return true
+  else return false
+}
+const DateToDay = (dd, mm, yyyy) => {
+  return Math.floor((365 * (yyyy - 1)) + ((yyyy - 1) / 4) - ((yyyy - 1) / 100) + ((yyyy - 1) / 400) + dd + (((367 * mm) - 362) / 12) + (mm <= 2 ? 0 : isLeap(yyyy) ? -1 : -2))
+}
+
+const DateDayDifference = (date1, date2) => {
+  // firstly, check that both input are string or not.
+  if (typeof date1 !== 'string' && typeof date2 !== 'string') {
+    return new TypeError('Argument is not a string.')
+  }
+  // extarct the first date
+  const [firstDateDay, firstDateMonth, firstDateYear] = date1.split('/').map((ele) => Number(ele))
+  // extarct the second date
+  const [secondDateDay, secondDateMonth, secondDateYear] = date2.split('/').map((ele) => Number(ele))
+  // check the both data are valid or not.
+  if (firstDateDay < 0 || firstDateDay > 31 ||
+        firstDateMonth > 12 || firstDateMonth < 0 ||
+        secondDateDay < 0 || secondDateDay > 31 ||
+        secondDateMonth > 12 || secondDateMonth < 0) {
+    return new TypeError('Date is not valid.')
+  }
+  return Math.abs(DateToDay(secondDateDay, secondDateMonth, secondDateYear) - DateToDay(firstDateDay, firstDateMonth, firstDateYear))
+}
+
+// Example : DateDayDifference('17/08/2002', '10/10/2020') => 6630
+
+module.exports = DateDayDifference