Fix differenceInDays across DST (closes #533) (#1630)

src/differenceInDays/index.js

@@ -1,6 +1,4 @@
 import toDate from '../toDate/index.js'
-import differenceInCalendarDays from '../differenceInCalendarDays/index.js'
-import compareAsc from '../compareAsc/index.js'
 import requiredArgs from '../_lib/requiredArgs/index.js'
 
 /**
@@ -10,6 +8,8 @@ import requiredArgs from '../_lib/requiredArgs/index.js'
  *
  * @description
  * Get the number of full day periods between the given dates.
+ * This function returns the difference in days as an integer whole number of 24 hour periods between two timestamps,  and thereby ignores DST changes.
+ *
  *
  * ### v2.0.0 breaking changes:
  *
@@ -36,21 +36,17 @@ import requiredArgs from '../_lib/requiredArgs/index.js'
  * )
  * //=> 0
  */
+var MILLISECONDS_IN_DAY = 86400000
+
 export default function differenceInDays(dirtyDateLeft, dirtyDateRight) {
   requiredArgs(2, arguments)
 
   var dateLeft = toDate(dirtyDateLeft)
   var dateRight = toDate(dirtyDateRight)
 
-  var sign = compareAsc(dateLeft, dateRight)
-  var difference = Math.abs(differenceInCalendarDays(dateLeft, dateRight))
-
-  dateLeft.setDate(dateLeft.getDate() - sign * difference)
+  var result = (dateLeft - dateRight) / MILLISECONDS_IN_DAY
 
-  // 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
-  var isLastDayNotFull = compareAsc(dateLeft, dateRight) === -sign
-  var result = sign * (difference - isLastDayNotFull)
-  // Prevent negative zero
-  return result === 0 ? 0 : result
+  // round towards zero
+  if (result > 0) return Math.floor(result)
+  return Math.ceil(result)
 }

src/differenceInDays/test.js

@@ -62,6 +62,16 @@ describe('differenceInDays', function() {
       assert(result === 0)
     })
 
+    it('ignores any localtime differences with DST', function() {
+      const a = new Date('2019-10-05T15:00:00.000Z') // for TZ=Australia/Sydney, this is 1 hour before DST
+      const b = new Date('2019-10-06T14:00:00.000Z') // for TZ=Australia/Sydney, this is 1 day  after DST
+      const c = new Date('2019-10-07T14:00:00.000Z') // for TZ=Australia/Sydney, this is 2 days after DST
+
+      assert(differenceInDays(c, b) === 1)
+      assert(differenceInDays(b, a) === 0) // 23 hours < 1 day
+      assert(differenceInDays(c, a) === 1) // 47 hours < 2 days
+    })
+
     it('does not return -0 when the given dates are the same', () => {
       function isNegativeZero(x) {
         return x === 0 && 1 / x < 0