1 files changed, 104 insertions, 0 deletions
diff --git a/date-fns/src/differenceInDays/index.ts b/date-fns/src/differenceInDays/index.ts
new file mode 100644
index 0000000..5851836
--- /dev/null
+++ b/date-fns/src/differenceInDays/index.ts
@@ -0,0 +1,104 @@
+import toDate from '../toDate/index'
+import differenceInCalendarDays from '../differenceInCalendarDays/index'
+import requiredArgs from '../_lib/requiredArgs/index'
+
+// Like `compareAsc` but uses local time not UTC, which is needed
+// for accurate equality comparisons of UTC timestamps that end up
+// having the same representation in local time, e.g. one hour before
+// DST ends vs. the instant that DST ends.
+function compareLocalAsc(dateLeft: Date, dateRight: Date): number {
+  const diff =
+    dateLeft.getFullYear() - dateRight.getFullYear() ||
+    dateLeft.getMonth() - dateRight.getMonth() ||
+    dateLeft.getDate() - dateRight.getDate() ||
+    dateLeft.getHours() - dateRight.getHours() ||
+    dateLeft.getMinutes() - dateRight.getMinutes() ||
+    dateLeft.getSeconds() - dateRight.getSeconds() ||
+    dateLeft.getMilliseconds() - dateRight.getMilliseconds()
+
+  if (diff < 0) {
+    return -1
+  } else if (diff > 0) {
+    return 1
+    // Return 0 if diff is 0; return NaN if diff is NaN
+  } else {
+    return diff
+  }
+}
+
+/**
+ * @name differenceInDays
+ * @category Day Helpers
+ * @summary Get the number of full days between the given dates.
+ *
+ * @description
+ * Get the number of full day periods between two dates. Fractional days are
+ * truncated towards zero.
+ *
+ * One "full day" is the distance between a local time in one day to the same
+ * local time on the next or previous day. A full day can sometimes be less than
+ * or more than 24 hours if a daylight savings change happens between two dates.
+ *
+ * To ignore DST and only measure exact 24-hour periods, use this instead:
+ * `Math.floor(differenceInHours(dateLeft, dateRight)/24)|0`.
+ *
+ *
+ * ### 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 days according to the local timezone
+ * @throws {TypeError} 2 arguments required
+ *
+ * @example
+ * // How many full days are between
+ * // 2 July 2011 23:00:00 and 2 July 2012 00:00:00?
+ * const result = differenceInDays(
+ *   new Date(2012, 6, 2, 0, 0),
+ *   new Date(2011, 6, 2, 23, 0)
+ * )
+ * //=> 365
+ * // How many full days are between
+ * // 2 July 2011 23:59:00 and 3 July 2011 00:01:00?
+ * const result = differenceInDays(
+ *   new Date(2011, 6, 3, 0, 1),
+ *   new Date(2011, 6, 2, 23, 59)
+ * )
+ * //=> 0
+ * // How many full days are between
+ * // 1 March 2020 0:00 and 1 June 2020 0:00 ?
+ * // Note: because local time is used, the
+ * // result will always be 92 days, even in
+ * // time zones where DST starts and the
+ * // period has only 92*24-1 hours.
+ * const result = differenceInDays(
+ *   new Date(2020, 5, 1),
+ *   new Date(2020, 2, 1)
+ * )
+//=> 92
+ */
+export default function differenceInDays(
+  dirtyDateLeft: Date | number,
+  dirtyDateRight: Date | number
+): number {
+  requiredArgs(2, arguments)
+
+  const dateLeft = toDate(dirtyDateLeft)
+  const dateRight = toDate(dirtyDateRight)
+
+  const sign = compareLocalAsc(dateLeft, dateRight)
+  const difference = Math.abs(differenceInCalendarDays(dateLeft, dateRight))
+
+  dateLeft.setDate(dateLeft.getDate() - sign * difference)
+
+  // Math.abs(diff in full days - diff in calendar days) === 1 if last calendar day is not full
+  // If so, result must be decreased by 1 in absolute value
+  const isLastDayNotFull = Number(
+    compareLocalAsc(dateLeft, dateRight) === -sign
+  )
+  const result = sign * (difference - isLastDayNotFull)
+  // Prevent negative zero
+  return result === 0 ? 0 : result
+}