Fix: _dts_in_same_interval("1mo") ignored year, treating same-month in different years as the same interval

The "1mo" branch compared only `dt1.month == dt2.month`, so e.g.
(2024-12-15, 2025-12-15) returned True. The "3mo" branch already
handles year differences via `year_diff`; mirror that by also
requiring `dt1.year == dt2.year`.

Added a test (`test_same_month_different_year`) that fails on the
old logic and passes on the fix. All existing tests still pass.

tests/test_utils.py

@@ -104,6 +104,17 @@ class TestDateIntervalCheck(unittest.TestCase):
         dt2 = pd.Timestamp("2025-01-15")
         self.assertFalse(_dts_in_same_interval(dt1, dt2, "1mo"))
 
+    def test_same_month_different_year(self):
+        # Same calendar month but different years must not be treated
+        # as the same monthly interval.
+        dt1 = pd.Timestamp("2024-12-15")
+        dt2 = pd.Timestamp("2025-12-15")
+        self.assertFalse(_dts_in_same_interval(dt1, dt2, "1mo"))
+
+        dt3 = pd.Timestamp("2020-06-01")
+        dt4 = pd.Timestamp("2025-06-01")
+        self.assertFalse(_dts_in_same_interval(dt3, dt4, "1mo"))
+
     def test_standard_quarters(self):
         q1_start = datetime(2023, 1, 1)
         self.assertTrue(_dts_in_same_interval(q1_start, datetime(2023, 1, 15), '3mo'))

yfinance/utils.py

@@ -624,7 +624,7 @@ def _dts_in_same_interval(dt1, dt2, interval):
     elif interval == "1wk":
         last_rows_same_interval = (dt2 - dt1).days < 7
     elif interval == "1mo":
-        last_rows_same_interval = dt1.month == dt2.month
+        last_rows_same_interval = dt1.month == dt2.month and dt1.year == dt2.year
     elif interval == "3mo":
         shift = (dt1.month % 3) - 1
         q1 = (dt1.month - shift - 1) // 3 + 1