Feat(datetime utility): add diff months util

Add utility to get difference in months

app/assets/javascripts/lib/utils/datetime_utility.js

@@ -744,6 +744,21 @@ export const differenceInSeconds = (startDate, endDate) => {
   return (endDate.getTime() - startDate.getTime()) / 1000;
 };
 
+/**
+ * A utility function which computes the difference in months
+ * between 2 dates.
+ *
+ * @param {Date} startDate the start date
+ * @param {Date} endDate the end date
+ *
+ * @return {Int} the difference in months
+ */
+export const differenceInMonths = (startDate, endDate) => {
+  const yearDiff = endDate.getYear() - startDate.getYear();
+  const monthDiff = endDate.getMonth() - startDate.getMonth();
+  return monthDiff + 12 * yearDiff;
+};
+
 /**
  * A utility function which computes the difference in milliseconds
  * between 2 dates.

spec/frontend/lib/utils/datetime_utility_spec.js

@@ -682,6 +682,20 @@ describe('differenceInSeconds', () => {
   });
 });
 
+describe('differenceInMonths', () => {
+  const startDateTime = new Date('2019-07-17T00:00:00.000Z');
+
+  it.each`
+    startDate                               | endDate                                 | expected
+    ${startDateTime}                        | ${startDateTime}                        | ${0}
+    ${startDateTime}                        | ${new Date('2019-12-17T12:00:00.000Z')} | ${5}
+    ${startDateTime}                        | ${new Date('2021-02-18T00:00:00.000Z')} | ${19}
+    ${new Date('2021-02-18T00:00:00.000Z')} | ${startDateTime}                        | ${-19}
+  `('returns $expected for $endDate - $startDate', ({ startDate, endDate, expected }) => {
+    expect(datetimeUtility.differenceInMonths(startDate, endDate)).toBe(expected);
+  });
+});
+
 describe('differenceInMilliseconds', () => {
   const startDateTime = new Date('2019-07-17T00:00:00.000Z');