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diff --git a/date-fns/src/differenceInCalendarISOWeeks/index.ts b/date-fns/src/differenceInCalendarISOWeeks/index.ts
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+import getTimezoneOffsetInMilliseconds from '../_lib/getTimezoneOffsetInMilliseconds/index'
+import startOfISOWeek from '../startOfISOWeek/index'
+import requiredArgs from '../_lib/requiredArgs/index'
+
+const MILLISECONDS_IN_WEEK = 604800000
+
+/**
+ * @name differenceInCalendarISOWeeks
+ * @category ISO Week Helpers
+ * @summary Get the number of calendar ISO weeks between the given dates.
+ *
+ * @description
+ * Get the number of calendar ISO weeks between the given dates.
+ *
+ * ISO week-numbering year: http://en.wikipedia.org/wiki/ISO_week_date
+ *
+ * ### 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 calendar ISO weeks
+ * @throws {TypeError} 2 arguments required
+ *
+ * @example
+ * // How many calendar ISO weeks are between 6 July 2014 and 21 July 2014?
+ * const result = differenceInCalendarISOWeeks(
+ *   new Date(2014, 6, 21),
+ *   new Date(2014, 6, 6)
+ * )
+ * //=> 3
+ */
+export default function differenceInCalendarISOWeeks(
+  dirtyDateLeft: Date | number,
+  dirtyDateRight: Date | number
+): number {
+  requiredArgs(2, arguments)
+
+  const startOfISOWeekLeft = startOfISOWeek(dirtyDateLeft)
+  const startOfISOWeekRight = startOfISOWeek(dirtyDateRight)
+
+  const timestampLeft =
+    startOfISOWeekLeft.getTime() -
+    getTimezoneOffsetInMilliseconds(startOfISOWeekLeft)
+  const timestampRight =
+    startOfISOWeekRight.getTime() -
+    getTimezoneOffsetInMilliseconds(startOfISOWeekRight)
+
+  // Round the number of days to the nearest integer
+  // because the number of milliseconds in a week is not constant
+  // (e.g. it's different in the week of the daylight saving time clock shift)
+  return Math.round((timestampLeft - timestampRight) / MILLISECONDS_IN_WEEK)
+}