diff --git a/pandas/_libs/tslibs/c_timestamp.pyx b/pandas/_libs/tslibs/c_timestamp.pyx
index 6e6b809b9b5a6..ed1df5f4fa595 100644
--- a/pandas/_libs/tslibs/c_timestamp.pyx
+++ b/pandas/_libs/tslibs/c_timestamp.pyx
@@ -57,11 +57,12 @@ def integer_op_not_supported(obj):
     #  the caller; mypy finds this more palatable.
     cls = type(obj).__name__
 
+    # GH#30886 using an fstring raises SystemError
     int_addsub_msg = (
-        f"Addition/subtraction of integers and integer-arrays with {cls} is "
+        "Addition/subtraction of integers and integer-arrays with {cls} is "
         "no longer supported.  Instead of adding/subtracting `n`, "
         "use `n * obj.freq`"
-    )
+    ).format(cls=cls)
     return TypeError(int_addsub_msg)
 
 
diff --git a/pandas/tests/frame/test_analytics.py b/pandas/tests/frame/test_analytics.py
index 910230c737a2a..25b2997eb088f 100644
--- a/pandas/tests/frame/test_analytics.py
+++ b/pandas/tests/frame/test_analytics.py
@@ -823,6 +823,16 @@ def test_sum_bool(self, float_frame):
         bools.sum(1)
         bools.sum(0)
 
+    def test_sum_mixed_datetime(self):
+        # GH#30886
+        df = pd.DataFrame(
+            {"A": pd.date_range("2000", periods=4), "B": [1, 2, 3, 4]}
+        ).reindex([2, 3, 4])
+        result = df.sum()
+
+        expected = pd.Series({"B": 7.0})
+        tm.assert_series_equal(result, expected)
+
     def test_mean_corner(self, float_frame, float_string_frame):
         # unit test when have object data
         the_mean = float_string_frame.mean(axis=0)