5 files changed, 342 insertions, 0 deletions
diff --git a/date-fns/src/differenceInMonths/benchmark.js b/date-fns/src/differenceInMonths/benchmark.js
new file mode 100644
index 0000000..4c2f1c7
--- /dev/null
+++ b/date-fns/src/differenceInMonths/benchmark.js
@@ -0,0 +1,23 @@
+// @flow
+/* eslint-env mocha */
+/* global suite, benchmark */
+
+import differenceInMonths from '.'
+import moment from 'moment'
+
+suite('differenceInMonths', function () {
+  benchmark('date-fns', function () {
+    return differenceInMonths(this.dateA, this.dateB)
+  })
+
+  benchmark('Moment.js', function () {
+    return this.momentA.diff(this.momentB, 'month')
+  })
+}, {
+  setup: function () {
+    this.dateA = new Date()
+    this.momentA = moment()
+    this.dateB = new Date(this.dateA.getTime() + 604800000)
+    this.momentB = this.momentA.clone().add(7, 'days')
+  }
+})
diff --git a/date-fns/src/differenceInMonths/index.d.ts b/date-fns/src/differenceInMonths/index.d.ts
new file mode 100644
index 0000000..8fbe977
--- /dev/null
+++ b/date-fns/src/differenceInMonths/index.d.ts
@@ -0,0 +1,4 @@
+// This file is generated automatically by `scripts/build/typings.js`. Please, don't change it.
+
+import { differenceInMonths } from 'date-fns'
+export default differenceInMonths
diff --git a/date-fns/src/differenceInMonths/index.js.flow b/date-fns/src/differenceInMonths/index.js.flow
new file mode 100644
index 0000000..c799819
--- /dev/null
+++ b/date-fns/src/differenceInMonths/index.js.flow
@@ -0,0 +1,55 @@
+// @flow
+// This file is generated automatically by `scripts/build/typings.js`. Please, don't change it.
+
+export type Interval = {
+  start: Date | number,
+  end: Date | number,
+}
+
+export type Locale = {
+  code?: string,
+  formatDistance?: (...args: Array<any>) => any,
+  formatRelative?: (...args: Array<any>) => any,
+  localize?: {
+    ordinalNumber: (...args: Array<any>) => any,
+    era: (...args: Array<any>) => any,
+    quarter: (...args: Array<any>) => any,
+    month: (...args: Array<any>) => any,
+    day: (...args: Array<any>) => any,
+    dayPeriod: (...args: Array<any>) => any,
+  },
+  formatLong?: {
+    date: (...args: Array<any>) => any,
+    time: (...args: Array<any>) => any,
+    dateTime: (...args: Array<any>) => any,
+  },
+  match?: {
+    ordinalNumber: (...args: Array<any>) => any,
+    era: (...args: Array<any>) => any,
+    quarter: (...args: Array<any>) => any,
+    month: (...args: Array<any>) => any,
+    day: (...args: Array<any>) => any,
+    dayPeriod: (...args: Array<any>) => any,
+  },
+  options?: {
+    weekStartsOn?: 0 | 1 | 2 | 3 | 4 | 5 | 6,
+    firstWeekContainsDate?: 1 | 2 | 3 | 4 | 5 | 6 | 7,
+  },
+}
+
+export type Duration = {
+  years?: number,
+  months?: number,
+  weeks?: number,
+  days?: number,
+  hours?: number,
+  minutes?: number,
+  seconds?: number,
+}
+
+export type Day = 0 | 1 | 2 | 3 | 4 | 5 | 6
+
+declare module.exports: (
+  dateLeft: Date | number,
+  dateRight: Date | number
+) => number
diff --git a/date-fns/src/differenceInMonths/index.ts b/date-fns/src/differenceInMonths/index.ts
new file mode 100644
index 0000000..c8a5c14
--- /dev/null
+++ b/date-fns/src/differenceInMonths/index.ts
@@ -0,0 +1,72 @@
+import toDate from '../toDate/index'
+import differenceInCalendarMonths from '../differenceInCalendarMonths/index'
+import compareAsc from '../compareAsc/index'
+import requiredArgs from '../_lib/requiredArgs/index'
+import isLastDayOfMonth from '../isLastDayOfMonth/index'
+
+/**
+ * @name differenceInMonths
+ * @category Month Helpers
+ * @summary Get the number of full months between the given dates.
+ *
+ * @description
+ * Get the number of full months between the given dates.
+ *
+ * ### v2.0.0 breaking changes:
+ *
+ * - [Changes that are common for the whole library](https://github.com/date-fns/date-fns/blob/master/docs/upgradeGuide.md#Common-Changes).
+ *
+ * @param {Date|Number} dateLeft - the later date
+ * @param {Date|Number} dateRight - the earlier date
+ * @returns {Number} the number of full months
+ * @throws {TypeError} 2 arguments required
+ *
+ * @example
+ * // How many full months are between 31 January 2014 and 1 September 2014?
+ * var result = differenceInMonths(new Date(2014, 8, 1), new Date(2014, 0, 31))
+ * //=> 7
+ */
+export default function differenceInMonths(
+  dirtyDateLeft: Date | number,
+  dirtyDateRight: Date | number
+): number {
+  requiredArgs(2, arguments)
+
+  const dateLeft = toDate(dirtyDateLeft)
+  const dateRight = toDate(dirtyDateRight)
+
+  const sign = compareAsc(dateLeft, dateRight)
+  const difference = Math.abs(differenceInCalendarMonths(dateLeft, dateRight))
+  let result
+
+  // Check for the difference of less than month
+  if (difference < 1) {
+    result = 0
+  } else {
+    if (dateLeft.getMonth() === 1 && dateLeft.getDate() > 27) {
+      // This will check if the date is end of Feb and assign a higher end of month date
+      // to compare it with Jan
+      dateLeft.setDate(30)
+    }
+
+    dateLeft.setMonth(dateLeft.getMonth() - sign * difference)
+
+    // Math.abs(diff in full months - diff in calendar months) === 1 if last calendar month is not full
+    // If so, result must be decreased by 1 in absolute value
+    let isLastMonthNotFull = compareAsc(dateLeft, dateRight) === -sign
+
+    // Check for cases of one full calendar month
+    if (
+      isLastDayOfMonth(toDate(dirtyDateLeft)) &&
+      difference === 1 &&
+      compareAsc(dirtyDateLeft, dateRight) === 1
+    ) {
+      isLastMonthNotFull = false
+    }
+
+    result = sign * (difference - Number(isLastMonthNotFull))
+  }
+
+  // Prevent negative zero
+  return result === 0 ? 0 : result
+}
diff --git a/date-fns/src/differenceInMonths/test.ts b/date-fns/src/differenceInMonths/test.ts
new file mode 100644
index 0000000..edebb59
--- /dev/null
+++ b/date-fns/src/differenceInMonths/test.ts
@@ -0,0 +1,188 @@
+/* eslint-env mocha */
+
+import assert from 'power-assert'
+import differenceInMonths from '.'
+
+describe('differenceInMonths', function () {
+  it('returns the number of full months between the given dates', function () {
+    const result = differenceInMonths(
+      new Date(2012, 6 /* Jul */, 2, 18, 0),
+      new Date(2011, 6 /* Jul */, 2, 6, 0)
+    )
+    assert(result === 12)
+  })
+
+  it('returns a negative number if the time value of the first date is smaller', function () {
+    const result = differenceInMonths(
+      new Date(2011, 6 /* Jul */, 2, 6, 0),
+      new Date(2012, 6 /* Jul */, 2, 18, 0)
+    )
+    assert(result === -12)
+  })
+
+  it('accepts timestamps', function () {
+    const result = differenceInMonths(
+      new Date(2014, 7 /* Aug */, 2).getTime(),
+      new Date(2010, 6 /* Jul */, 2).getTime()
+    )
+    assert(result === 49)
+  })
+
+  describe('edge cases', function () {
+    it('it returns diff of 1 month between Feb 28 2021 and Jan 30 2021', function () {
+      const result = differenceInMonths(
+        new Date(2021, 1 /* Feb */, 28),
+        new Date(2021, 0 /* Jan */, 30)
+      )
+      assert(result === 1)
+    })
+
+    it('it returns diff of 1 month between Feb 28 2021 and Jan 31 2021', function () {
+      const result = differenceInMonths(
+        new Date(2021, 1 /* Feb */, 28),
+        new Date(2021, 0 /* Jan */, 31)
+      )
+      assert(result === 1)
+    })
+
+    it('it returns diff of 1 month between Nov, 30 2021 and Oct, 31 2021', function () {
+      const result = differenceInMonths(
+        new Date(2021, 10 /* Nov */, 30),
+        new Date(2021, 9 /* Oct */, 31)
+      )
+      assert(result === 1)
+    })
+
+    it('it returns diff of 1 month between Oct, 31 2021 and Sep, 30 2021', function () {
+      const result = differenceInMonths(
+        new Date(2021, 9 /* Oct */, 31),
+        new Date(2021, 8 /* Sep */, 30)
+      )
+      assert(result === 1)
+    })
+
+    it('it returns diff of 6 month between Oct, 31 2021 and Apr, 30 2021', function () {
+      const result = differenceInMonths(
+        new Date(2021, 9 /* Oct */, 31),
+        new Date(2021, 3 /* Apr */, 30)
+      )
+      assert(result === 6)
+    })
+
+    it('it returns diff of -1 month between Sep, 30 2021 and Oct, 31 2021', function () {
+      const result = differenceInMonths(
+        new Date(2021, 8 /* Sep */, 30),
+        new Date(2021, 9 /* Oct */, 31)
+      )
+      assert(result === -1)
+    })
+
+    it('the difference is less than a month, but the given dates are in different calendar months', function () {
+      const result = differenceInMonths(
+        new Date(2014, 7 /* Aug */, 1),
+        new Date(2014, 6 /* Jul */, 31)
+      )
+      assert(result === 0)
+    })
+
+    it('the same for the swapped dates', function () {
+      const result = differenceInMonths(
+        new Date(2014, 6 /* Jul */, 31),
+        new Date(2014, 7 /* Aug */, 1)
+      )
+      assert(result === 0)
+    })
+
+    it('the days of months of the given dates are the same', function () {
+      const result = differenceInMonths(
+        new Date(2014, 8 /* Sep */, 6),
+        new Date(2014, 7 /* Aug */, 6)
+      )
+      assert(result === 1)
+    })
+
+    it('the given dates are the same', function () {
+      const result = differenceInMonths(
+        new Date(2014, 8 /* Sep */, 5, 0, 0),
+        new Date(2014, 8 /* Sep */, 5, 0, 0)
+      )
+      assert(result === 0)
+    })
+
+    it('does not return -0 when the given dates are the same', () => {
+      function isNegativeZero(x: number): boolean {
+        return x === 0 && 1 / x < 0
+      }
+
+      const result = differenceInMonths(
+        new Date(2014, 8 /* Sep */, 5, 0, 0),
+        new Date(2014, 8 /* Sep */, 5, 0, 0)
+      )
+
+      const resultIsNegative = isNegativeZero(result)
+      assert(resultIsNegative === false)
+    })
+  })
+
+  it('returns NaN if the first date is `Invalid Date`', function () {
+    const result = differenceInMonths(
+      new Date(NaN),
+      new Date(2017, 0 /* Jan */, 1)
+    )
+    assert(isNaN(result))
+  })
+
+  it('returns NaN if the second date is `Invalid Date`', function () {
+    const result = differenceInMonths(
+      new Date(2017, 0 /* Jan */, 1),
+      new Date(NaN)
+    )
+    assert(isNaN(result))
+  })
+
+  it('returns NaN if the both dates are `Invalid Date`', function () {
+    const result = differenceInMonths(new Date(NaN), new Date(NaN))
+    assert(isNaN(result))
+  })
+
+  it('throws TypeError exception if passed less than 2 arguments', function () {
+    assert.throws(differenceInMonths.bind(null), TypeError)
+    assert.throws(differenceInMonths.bind(null, 1), TypeError)
+  })
+
+  describe('edge cases', function () {
+    it('returns the number of full months between the given dates - end of Feb', function () {
+      assert(
+        differenceInMonths(
+          new Date(2012, 1 /* Feb */, 29, 9, 0, 0),
+          new Date(2012, 1 /* Feb */, 29, 10, 0, 0)
+        ) === 0
+      )
+      assert(
+        differenceInMonths(
+          new Date(2012, 1 /* Feb */, 28, 9, 0, 0),
+          new Date(2012, 1 /* Feb */, 29, 10, 0, 0)
+        ) === 0
+      )
+      assert(
+        differenceInMonths(
+          new Date(2012, 1 /* Feb */, 27, 9, 0, 0),
+          new Date(2012, 1 /* Feb */, 27, 10, 0, 0)
+        ) === 0
+      )
+      assert(
+        differenceInMonths(
+          new Date(2012, 1 /* Feb */, 28, 9, 0, 0),
+          new Date(2012, 1 /* Feb */, 28, 10, 0, 0)
+        ) === 0
+      )
+    })
+
+    assert(
+      differenceInMonths(
+        new Date(2021, 1 /* Feb */, 28, 7, 23, 7),
+        new Date(2021, 1 /* Feb */, 28, 7, 38, 18)
+      ) === 0
+    )
+  })
+})