Merge pull request #4717 from asenousy:end-of-month-diff-bugs

[bugfix] More predictable month diffs at end-of-month

src/lib/moment/diff.js

@@ -52,6 +52,11 @@ export function diff(input, units, asFloat) {
 }
 
 function monthDiff(a, b) {
+    if (a.date() < b.date()) {
+        // end-of-month calculations work correct when the start month has more
+        // days than the end month.
+        return -monthDiff(b, a);
+    }
     // difference in months
     var wholeMonthDiff = (b.year() - a.year()) * 12 + (b.month() - a.month()),
         // b is in (anchor - 1 month, anchor + 1 month)

src/test/moment/diff.js

@@ -209,6 +209,39 @@ test('diff month', function (assert) {
     );
 });
 
+test('end of month diff', function (assert) {
+    assert.equal(
+        moment('2016-02-29').diff('2016-01-30', 'months'),
+        1,
+        'Feb 29 to Jan 30 should be 1 month'
+    );
+    assert.equal(
+        moment('2016-02-29').diff('2016-01-31', 'months'),
+        1,
+        'Feb 29 to Jan 31 should be 1 month'
+    );
+    assert.equal(
+        moment('2016-05-31')
+            .add(1, 'month')
+            .diff(moment('2016-05-31'), 'month'),
+        1,
+        '(May 31 plus 1 month) to May 31 should be 1 month diff'
+    );
+});
+
+test('end of month diff with time behind', function (assert) {
+    assert.equal(
+        moment('2017-03-31').diff('2017-02-28', 'months'),
+        1,
+        'Feb 28 to March 31 should be 1 month'
+    );
+    assert.equal(
+        moment('2017-02-28').diff('2017-03-31', 'months'),
+        -1,
+        'Feb 28 to March 31 should be 1 month'
+    );
+});
+
 test('diff across DST', function (assert) {
     var dst = dstForYear(2012),
         a,