Merge pull request #1 from akshat62/codex/fix-bug-related-to-issue-62094

Preserve day freq on DatetimeIndex subtraction

Author: akshat62

pandas/core/arrays/datetimelike.py

@@ -1089,6 +1089,16 @@ def _get_arithmetic_result_freq(self, other) -> BaseOffset | None:
             # e.g. TestTimedelta64ArithmeticUnsorted::test_timedelta
             # Day is unambiguously 24h
             return self.freq
+        elif (
+            lib.is_np_dtype(self.dtype, "M")
+            and isinstance(self.freq, Day)
+            and isinstance(other, Timestamp)
+            and (self.tz is None or timezones.is_utc(self.tz))
+            and (other.tz is None or timezones.is_utc(other.tz))
+        ):
+            # e.g. issue gh-62094: subtracting a Timestamp from a DTI
+            # with Day freq retains that freq
+            return self.freq
 
         return None
 

pandas/tests/indexes/timedeltas/methods/test_shift.py

@@ -74,3 +74,10 @@ def test_shift_no_freq(self):
         tdi = TimedeltaIndex(["1 days 01:00:00", "2 days 01:00:00"], freq=None)
         with pytest.raises(NullFrequencyError, match="Cannot shift with no freq"):
             tdi.shift(2)
+
+    def test_shift_after_dti_sub_timestamp(self):
+        dti = pd.date_range("1/1/2021", "1/5/2021")
+        tdi = dti - pd.Timestamp("1/3/2019")
+        result = tdi.shift(1)
+        expected = tdi + pd.Timedelta(days=1)
+        tm.assert_index_equal(result, expected)