From 770b533d3e4e09e6e191ed609cab89049d7a07a2 Mon Sep 17 00:00:00 2001
From: Brock <jbrockmendel@gmail.com>
Date: Fri, 30 Sep 2022 11:20:20 -0700
Subject: [PATCH] BUG: DatetimeArray-datetimelike mixed resos

---
 pandas/_libs/tslibs/timestamps.pyi    |  1 +
 pandas/core/arrays/datetimelike.py    | 20 ++++++++++++++++-
 pandas/tests/arrays/test_datetimes.py | 31 ++++++++++++++++++++++++++-
 3 files changed, 50 insertions(+), 2 deletions(-)

diff --git a/pandas/_libs/tslibs/timestamps.pyi b/pandas/_libs/tslibs/timestamps.pyi
index e4be7fda43005..8382fe0274138 100644
--- a/pandas/_libs/tslibs/timestamps.pyi
+++ b/pandas/_libs/tslibs/timestamps.pyi
@@ -27,6 +27,7 @@ _DatetimeT = TypeVar("_DatetimeT", bound=datetime)
 def integer_op_not_supported(obj: object) -> TypeError: ...
 
 class Timestamp(datetime):
+    _reso: int
     min: ClassVar[Timestamp]
     max: ClassVar[Timestamp]
 
diff --git a/pandas/core/arrays/datetimelike.py b/pandas/core/arrays/datetimelike.py
index 71f834e0f4e01..c42eeb9435c89 100644
--- a/pandas/core/arrays/datetimelike.py
+++ b/pandas/core/arrays/datetimelike.py
@@ -1218,6 +1218,13 @@ def _sub_datetimelike_scalar(self, other: datetime | np.datetime64):
             new_message = str(err).replace("compare", "subtract")
             raise type(err)(new_message) from err
 
+        if other._reso != self._reso:
+            if other._reso < self._reso:
+                other = other._as_unit(self._unit)
+            else:
+                unit = npy_unit_to_abbrev(other._reso)
+                self = self._as_unit(unit)
+
         i8 = self.asi8
         result = checked_add_with_arr(i8, -other.value, arr_mask=self._isnan)
         res_m8 = result.view(f"timedelta64[{self._unit}]")
@@ -1248,12 +1255,23 @@ def _sub_datetime_arraylike(self, other):
             new_message = str(err).replace("compare", "subtract")
             raise type(err)(new_message) from err
 
+        if other._reso != self._reso:
+            if other._reso < self._reso:
+                other = other._as_unit(self._unit)
+            else:
+                self = self._as_unit(other._unit)
+
         self_i8 = self.asi8
         other_i8 = other.asi8
         new_values = checked_add_with_arr(
             self_i8, -other_i8, arr_mask=self._isnan, b_mask=other._isnan
         )
-        return new_values.view("timedelta64[ns]")
+        res_m8 = new_values.view(f"timedelta64[{self._unit}]")
+
+        from pandas.core.arrays import TimedeltaArray
+
+        new_freq = self._get_arithmetic_result_freq(other)
+        return TimedeltaArray._simple_new(res_m8, dtype=res_m8.dtype, freq=new_freq)
 
     @final
     def _sub_period(self, other: Period) -> npt.NDArray[np.object_]:
diff --git a/pandas/tests/arrays/test_datetimes.py b/pandas/tests/arrays/test_datetimes.py
index 83850c6cd2594..8c2a8df7c2fbd 100644
--- a/pandas/tests/arrays/test_datetimes.py
+++ b/pandas/tests/arrays/test_datetimes.py
@@ -13,7 +13,10 @@
 
 import pandas as pd
 import pandas._testing as tm
-from pandas.core.arrays import DatetimeArray
+from pandas.core.arrays import (
+    DatetimeArray,
+    TimedeltaArray,
+)
 
 
 class TestNonNano:
@@ -218,6 +221,32 @@ def test_add_mismatched_reso_doesnt_downcast(self):
         #  (so we _could_ downcast to unit="s"), we do not.
         assert res._unit == "us"
 
+    def test_sub_datetimelike_scalar_mismatch(self):
+        dti = pd.date_range("2016-01-01", periods=3)
+        dta = dti._data._as_unit("us")
+
+        ts = dta[0]._as_unit("s")
+
+        result = dta - ts
+        expected = (dti - dti[0])._data._as_unit("us")
+        assert result.dtype == "m8[us]"
+        tm.assert_extension_array_equal(result, expected)
+
+    def test_sub_datetime64_reso_mismatch(self):
+        dti = pd.date_range("2016-01-01", periods=3)
+        left = dti._data._as_unit("s")
+        right = left._as_unit("ms")
+
+        result = left - right
+        exp_values = np.array([0, 0, 0], dtype="m8[ms]")
+        expected = TimedeltaArray._simple_new(
+            exp_values,
+            dtype=exp_values.dtype,
+        )
+        tm.assert_extension_array_equal(result, expected)
+        result2 = right - left
+        tm.assert_extension_array_equal(result2, expected)
+
 
 class TestDatetimeArrayComparisons:
     # TODO: merge this into tests/arithmetic/test_datetime64 once it is