BUG: can't round-trip non-nano Timestamp

pandas/_libs/tslibs/conversion.pyx

@@ -294,9 +294,21 @@ cdef _TSObject convert_to_tsobject(object ts, tzinfo tz, str unit,
         if ts == NPY_NAT:
             obj.value = NPY_NAT
         else:
-            ts = cast_from_unit(ts, unit)
-            obj.value = ts
-            pandas_datetime_to_datetimestruct(ts, NPY_FR_ns, &obj.dts)
+            if unit is None:
+                in_reso = abbrev_to_npy_unit("ns")
+            else:
+                in_reso = abbrev_to_npy_unit(unit)
+            reso = get_supported_reso(in_reso)
+            try:
+                value = convert_reso(ts, in_reso, reso, False)
+            except OverflowError:
+                # GH#26651 re-raise as OutOfBoundsDatetime
+                raise OutOfBoundsDatetime(
+                    f"cannot convert input {ts} with the unit '{unit}'"
+                )
+            obj.value = value
+            obj.creso = reso
+            pandas_datetime_to_datetimestruct(value, reso, &obj.dts)
     elif is_float_object(ts):
         if ts != ts or ts == NPY_NAT:
             obj.value = NPY_NAT

pandas/tests/scalar/timestamp/test_constructors.py

@@ -76,7 +76,7 @@ def test_constructor_from_date_second_reso(self):
     @pytest.mark.parametrize("typ", [int, float])
     def test_construct_from_int_float_with_unit_out_of_bound_raises(self, typ):
         # GH#50870  make sure we get a OutOfBoundsDatetime instead of OverflowError
-        val = typ(150000000)
+        val = typ(150000000000000)
 
         msg = f"cannot convert input {val} with the unit 'D'"
         with pytest.raises(OutOfBoundsDatetime, match=msg):

pandas/tests/scalar/timestamp/test_timestamp.py

@@ -996,6 +996,12 @@ def test_resolution(self, ts):
         assert result == expected
         assert result._creso == expected._creso
 
+    def test_out_of_ns_bounds(self):
+        # https://github.com/pandas-dev/pandas/issues/51060
+        result = Timestamp(-52700112000, unit="s")
+        assert result == Timestamp("0300-01-01")
+        assert result.to_numpy() == np.datetime64("0300-01-01T00:00:00", "s")
+
 
 def test_timestamp_class_min_max_resolution():
     # when accessed on the class (as opposed to an instance), we default

pandas/tests/tools/test_to_datetime.py

@@ -1765,7 +1765,7 @@ def test_unit_array_mixed_nans(self, cache):
         result = to_datetime(values, unit="D", errors="ignore", cache=cache)
         expected = Index(
             [
-                11111111,
+                Timestamp(11111111, unit="D"),
                 Timestamp("1970-01-02"),
                 Timestamp("1970-01-02"),
                 NaT,
@@ -1792,7 +1792,9 @@ def test_unit_array_mixed_nans_large_int(self, cache):
         values = [1420043460000, iNaT, NaT, np.nan, "NaT"]
 
         result = to_datetime(values, errors="ignore", unit="s", cache=cache)
-        expected = Index([1420043460000, NaT, NaT, NaT, NaT], dtype=object)
+        expected = Index(
+            [Timestamp(1420043460000, unit="s"), NaT, NaT, NaT, NaT], dtype=object
+        )
         tm.assert_index_equal(result, expected)
 
         result = to_datetime(values, errors="coerce", unit="s", cache=cache)