From cd0dd3ab5264836b2ef5c27c9596b9dd7ff4eeb5 Mon Sep 17 00:00:00 2001
From: iabhi4 <iamonecool@gmail.com>
Date: Thu, 26 Jun 2025 23:13:42 -0700
Subject: [PATCH] BUG: Raise OutOfBoundsDatetime when replacing with
 out-of-bounds datetime64[ns] (GH#61671)

---
 doc/source/whatsnew/v3.0.0.rst             |  1 +
 pandas/core/dtypes/cast.py                 | 19 +++++++++++++++++++
 pandas/tests/frame/methods/test_replace.py | 12 ++++++++++++
 3 files changed, 32 insertions(+)

diff --git a/doc/source/whatsnew/v3.0.0.rst b/doc/source/whatsnew/v3.0.0.rst
index 099e5bc48353a..c238f29e87c62 100644
--- a/doc/source/whatsnew/v3.0.0.rst
+++ b/doc/source/whatsnew/v3.0.0.rst
@@ -685,6 +685,7 @@ Datetimelike
 - Bug in :func:`tseries.frequencies.to_offset` would fail to parse frequency strings starting with "LWOM" (:issue:`59218`)
 - Bug in :meth:`DataFrame.fillna` raising an ``AssertionError`` instead of ``OutOfBoundsDatetime`` when filling a ``datetime64[ns]`` column with an out-of-bounds timestamp. Now correctly raises ``OutOfBoundsDatetime``. (:issue:`61208`)
 - Bug in :meth:`DataFrame.min` and :meth:`DataFrame.max` casting ``datetime64`` and ``timedelta64`` columns to ``float64`` and losing precision (:issue:`60850`)
+- Bug in :meth:`DataFrame.replace` where attempting to replace a ``datetime64[ns]`` column with an out-of-bounds timestamp would raise an ``AssertionError`` or silently coerce. Now correctly raises ``OutOfBoundsDatetime``. (:issue:`61671`)
 - Bug in :meth:`Dataframe.agg` with df with missing values resulting in IndexError (:issue:`58810`)
 - Bug in :meth:`DatetimeIndex.is_year_start` and :meth:`DatetimeIndex.is_quarter_start` does not raise on Custom business days frequencies bigger then "1C" (:issue:`58664`)
 - Bug in :meth:`DatetimeIndex.is_year_start` and :meth:`DatetimeIndex.is_quarter_start` returning ``False`` on double-digit frequencies (:issue:`58523`)
diff --git a/pandas/core/dtypes/cast.py b/pandas/core/dtypes/cast.py
index dae04ba6244d4..7e6900b03bf4b 100644
--- a/pandas/core/dtypes/cast.py
+++ b/pandas/core/dtypes/cast.py
@@ -53,6 +53,7 @@
     ensure_str,
     is_bool,
     is_complex,
+    is_datetime64_dtype,
     is_float,
     is_integer,
     is_object_dtype,
@@ -1358,6 +1359,24 @@ def find_result_type(left_dtype: DtypeObj, right: Any) -> DtypeObj:
         dtype, _ = infer_dtype_from(right)
         new_dtype = find_common_type([left_dtype, dtype])
 
+        # special case: datetime64[ns] inferred but the value may be out of bounds
+        if (
+            is_datetime64_dtype(new_dtype)
+            and lib.is_scalar(right)
+            and isinstance(right, (np.datetime64, dt.datetime, Timestamp))
+        ):
+            try:
+                ts = Timestamp(right)
+                casted = np.datetime64(ts, "ns")
+                if Timestamp(casted) != ts:
+                    raise OutOfBoundsDatetime(
+                        f"{right!r} overflows datetime64[ns] during dtype inference"
+                    )
+            except (OverflowError, ValueError) as e:
+                raise OutOfBoundsDatetime(
+                    f"Cannot safely store {right!r} in inferred dtype 'datetime64[ns]'"
+                ) from e
+
     return new_dtype
 
 
diff --git a/pandas/tests/frame/methods/test_replace.py b/pandas/tests/frame/methods/test_replace.py
index 9e302dc5f94ee..442055d7bc061 100644
--- a/pandas/tests/frame/methods/test_replace.py
+++ b/pandas/tests/frame/methods/test_replace.py
@@ -1430,6 +1430,18 @@ def test_replace_with_nil_na(self):
         result = ser.replace("nil", "anything else")
         tm.assert_frame_equal(expected, result)
 
+    def test_replace_outofbounds_datetime_raises(self):
+        # GH 61671
+        df = DataFrame([np.nan], dtype="datetime64[ns]")
+        too_big = datetime(3000, 1, 1)
+        from pandas.errors import OutOfBoundsDatetime
+
+        with pytest.raises(
+            OutOfBoundsDatetime,
+            match="Cannot safely store .* in inferred dtype 'datetime64\\[ns\\]'",
+        ):
+            df.replace(np.nan, too_big)
+
 
 class TestDataFrameReplaceRegex:
     @pytest.mark.parametrize(