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diff --git a/date-fns/src/getDaysInYear/index.ts b/date-fns/src/getDaysInYear/index.ts
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+++ b/date-fns/src/getDaysInYear/index.ts
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+import toDate from '../toDate/index'
+import isLeapYear from '../isLeapYear/index'
+import requiredArgs from '../_lib/requiredArgs/index'
+
+/**
+ * @name getDaysInYear
+ * @category Year Helpers
+ * @summary Get the number of days in a year of the given date.
+ *
+ * @description
+ * Get the number of days in a year of the given 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 number of days in a year
+ * @throws {TypeError} 1 argument required
+ *
+ * @example
+ * // How many days are in 2012?
+ * const result = getDaysInYear(new Date(2012, 0, 1))
+ * //=> 366
+ */
+export default function getDaysInYear(dirtyDate: Date | number): number {
+  requiredArgs(1, arguments)
+
+  const date = toDate(dirtyDate)
+
+  if (String(new Date(date)) === 'Invalid Date') {
+    return NaN
+  }
+
+  return isLeapYear(date) ? 366 : 365
+}