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diff --git a/date-fns/src/getISOWeek/index.ts b/date-fns/src/getISOWeek/index.ts
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+import toDate from '../toDate/index'
+import startOfISOWeek from '../startOfISOWeek/index'
+import startOfISOWeekYear from '../startOfISOWeekYear/index'
+import requiredArgs from '../_lib/requiredArgs/index'
+
+const MILLISECONDS_IN_WEEK = 604800000
+
+/**
+ * @name getISOWeek
+ * @category ISO Week Helpers
+ * @summary Get the ISO week of the given date.
+ *
+ * @description
+ * Get the ISO week of the given date.
+ *
+ * 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} date - the given date
+ * @returns {Number} the ISO week
+ * @throws {TypeError} 1 argument required
+ *
+ * @example
+ * // Which week of the ISO-week numbering year is 2 January 2005?
+ * const result = getISOWeek(new Date(2005, 0, 2))
+ * //=> 53
+ */
+export default function getISOWeek(dirtyDate: Date | number): number {
+  requiredArgs(1, arguments)
+
+  const date = toDate(dirtyDate)
+  const diff = startOfISOWeek(date).getTime() - startOfISOWeekYear(date).getTime()
+
+  // 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(diff / MILLISECONDS_IN_WEEK) + 1
+}