Use calendar months for humanize month and quarter granularity

arrow/arrow.py

@@ -1276,10 +1276,25 @@ def humanize(
                     delta = sign * delta_second / self._SECS_PER_DAY
                 elif granularity == "week":
                     delta = sign * delta_second / self._SECS_PER_WEEK
-                elif granularity == "month":
-                    delta = sign * delta_second / self._SECS_PER_MONTH
-                elif granularity == "quarter":
-                    delta = sign * delta_second / self._SECS_PER_QUARTER
+                elif granularity in ("month", "quarter"):
+                    # Count calendar months, as the "auto" granularity does, so
+                    # that e.g. Jan 1 to Mar 1 is 2 months rather than 59 / 30.5.
+                    # The partial month is a fraction of the next calendar month,
+                    # which keeps it below 1 and keeps the sign of a 0 delta.
+                    start, end = sorted((self._datetime, dt))
+                    calendar_diff = relativedelta(end, start)
+                    calendar_months = (
+                        calendar_diff.years * self._MONTHS_PER_YEAR
+                        + calendar_diff.months
+                    )
+                    month_start = start + relativedelta(months=calendar_months)
+                    month_end = start + relativedelta(months=calendar_months + 1)
+                    elapsed_months = calendar_months + (end - month_start) / (
+                        month_end - month_start
+                    )
+                    if granularity == "quarter":
+                        elapsed_months /= self._MONTHS_PER_QUARTER
+                    delta = sign * elapsed_months
                 elif granularity == "year":
                     delta = sign * delta_second / self._SECS_PER_YEAR
                 else:

tests/test_arrow.py

@@ -1991,8 +1991,9 @@ def test_granularity(self):
         later506 = self.now.shift(seconds=50 * 10**6)
         assert self.now.humanize(later506, granularity="week") == "82 weeks ago"
         assert later506.humanize(self.now, granularity="week") == "in 82 weeks"
-        assert self.now.humanize(later506, granularity="month") == "18 months ago"
-        assert later506.humanize(self.now, granularity="month") == "in 18 months"
+        # 2013-01-01 to 2014-08-02 is 19 calendar months
+        assert self.now.humanize(later506, granularity="month") == "19 months ago"
+        assert later506.humanize(self.now, granularity="month") == "in 19 months"
         assert self.now.humanize(later506, granularity="quarter") == "6 quarters ago"
         assert later506.humanize(self.now, granularity="quarter") == "in 6 quarters"
         assert self.now.humanize(later506, granularity="year") == "a year ago"
@@ -2037,6 +2038,32 @@ def test_granularity(self):
         with pytest.raises(ValueError):
             self.now.humanize(later108, granularity="years")
 
+    def test_granularity_calendar_months(self):
+        start = arrow.Arrow(2023, 1, 31)
+
+        for months, expected in [(1, "a month"), (2, "2 months"), (6, "6 months")]:
+            end = start.shift(months=months)
+            assert end.humanize(start, granularity="month") == f"in {expected}"
+            assert start.humanize(end, granularity="month") == f"{expected} ago"
+
+        for months, expected in [
+            (3, "a quarter"),
+            (6, "2 quarters"),
+            (18, "6 quarters"),
+        ]:
+            end = start.shift(months=months)
+            assert end.humanize(start, granularity="quarter") == f"in {expected}"
+            assert start.humanize(end, granularity="quarter") == f"{expected} ago"
+
+        # Less than a calendar month, even though it is more than 30.5 days
+        end = arrow.Arrow(2023, 1, 31, 23, 59)
+        assert end.humanize(arrow.Arrow(2023, 1, 1), granularity="month") == (
+            "in 0 months"
+        )
+        assert arrow.Arrow(2023, 1, 1).humanize(end, granularity="month") == (
+            "0 months ago"
+        )
+
     def test_multiple_granularity(self):
         assert self.now.humanize(granularity="second") == "just now"
         assert self.now.humanize(granularity=["second"]) == "just now"