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+import getISOWeekYear from '../getISOWeekYear/index'
+import startOfISOWeek from '../startOfISOWeek/index'
+import requiredArgs from '../_lib/requiredArgs/index'
+
+/**
+ * @name startOfISOWeekYear
+ * @category ISO Week-Numbering Year Helpers
+ * @summary Return the start of an ISO week-numbering year for the given date.
+ *
+ * @description
+ * Return the start of an ISO week-numbering year,
+ * which always starts 3 days before the year's first Thursday.
+ * The result will be in the local timezone.
+ *
+ * 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 original date
+ * @returns {Date} the start of an ISO week-numbering year
+ * @throws {TypeError} 1 argument required
+ *
+ * @example
+ * // The start of an ISO week-numbering year for 2 July 2005:
+ * const result = startOfISOWeekYear(new Date(2005, 6, 2))
+ * //=> Mon Jan 03 2005 00:00:00
+ */
+export default function startOfISOWeekYear(dirtyDate: Date | number): Date {
+  requiredArgs(1, arguments)
+
+  const year = getISOWeekYear(dirtyDate)
+  const fourthOfJanuary = new Date(0)
+  fourthOfJanuary.setFullYear(year, 0, 4)
+  fourthOfJanuary.setHours(0, 0, 0, 0)
+  const date = startOfISOWeek(fourthOfJanuary)
+  return date
+}