Improve to_datetime bounds checking (#50183)

* add test for float to_datetime near overflow bounds

* fix float to_datetime near overflow bounds

* fix typo and formatting

* fix formatting

* fix test to not fail on rounding differences

* don't use approximate comparison on datetimes, it doesn't work

* also can't convert datetime to float

* match dtypes

* TST: don't try to use non-integer years (see #50301)

* TST: don't cross an integer

(tsmax_in_days happens to be close to an integer,
and this is a test of rounding)

* PERF: remove unnecessary copy

* add whatsnew

doc/source/whatsnew/v2.0.0.rst

@@ -950,6 +950,7 @@ Datetimelike
 - Bug in :func:`to_datetime` was failing to parse date strings ``'today'`` and ``'now'`` if ``format`` was not ISO8601 (:issue:`50359`)
 - Bug in :func:`Timestamp.utctimetuple` raising a ``TypeError`` (:issue:`32174`)
 - Bug in :func:`to_datetime` was raising ``ValueError`` when parsing mixed-offset :class:`Timestamp` with ``errors='ignore'`` (:issue:`50585`)
+- Bug in :func:`to_datetime` was incorrectly handling floating-point inputs within 1 ``unit`` of the overflow boundaries (:issue:`50183`)
 
 Timedelta
 ^^^^^^^^^

pandas/core/tools/datetimes.py

@@ -514,11 +514,9 @@ def _to_datetime_with_unit(arg, unit, name, utc: bool, errors: str) -> Index:
         elif arg.dtype.kind == "f":
             mult, _ = precision_from_unit(unit)
 
-            iresult = arg.astype("i8")
             mask = np.isnan(arg) | (arg == iNaT)
-            iresult[mask] = 0
-
-            fvalues = iresult.astype("f8") * mult
+            fvalues = (arg * mult).astype("f8", copy=False)
+            fvalues[mask] = 0
 
             if (fvalues < Timestamp.min.value).any() or (
                 fvalues > Timestamp.max.value
@@ -528,11 +526,7 @@ def _to_datetime_with_unit(arg, unit, name, utc: bool, errors: str) -> Index:
                     return _to_datetime_with_unit(arg, unit, name, utc, errors)
                 raise OutOfBoundsDatetime(f"cannot convert input with unit '{unit}'")
 
-            # TODO: is fresult meaningfully different from fvalues?
-            fresult = (arg * mult).astype("f8")
-            fresult[mask] = 0
-
-            arr = fresult.astype("M8[ns]", copy=False)
+            arr = fvalues.astype("M8[ns]", copy=False)
             arr[mask] = np.datetime64("NaT", "ns")
 
             tz_parsed = None

pandas/tests/tools/test_to_datetime.py

@@ -1935,6 +1935,27 @@ def test_to_timestamp_unit_coerce(self, bad_val):
         result = to_datetime([1, 2, bad_val], unit="D", errors="coerce")
         tm.assert_index_equal(result, expected)
 
+    def test_float_to_datetime_raise_near_bounds(self):
+        # GH50183
+        msg = "cannot convert input with unit 'D'"
+        oneday_in_ns = 1e9 * 60 * 60 * 24
+        tsmax_in_days = 2**63 / oneday_in_ns  # 2**63 ns, in days
+        # just in bounds
+        should_succeed = Series(
+            [0, tsmax_in_days - 0.005, -tsmax_in_days + 0.005], dtype=float
+        )
+        expected = (should_succeed * oneday_in_ns).astype(np.int64)
+        for error_mode in ["raise", "coerce", "ignore"]:
+            result1 = to_datetime(should_succeed, unit="D", errors=error_mode)
+            tm.assert_almost_equal(result1.astype(np.int64), expected, rtol=1e-10)
+        # just out of bounds
+        should_fail1 = Series([0, tsmax_in_days + 0.005], dtype=float)
+        should_fail2 = Series([0, -tsmax_in_days - 0.005], dtype=float)
+        with pytest.raises(OutOfBoundsDatetime, match=msg):
+            to_datetime(should_fail1, unit="D", errors="raise")
+        with pytest.raises(OutOfBoundsDatetime, match=msg):
+            to_datetime(should_fail2, unit="D", errors="raise")
+
 
 class TestToDatetimeDataFrame:
     @pytest.fixture