Support negative values in fromtimestamp on Windows using 0 + timedelta.

Author: jhohm

Lib/_pydatetime.py

@@ -1037,6 +1037,11 @@ def fromtimestamp(cls, t):
         "Construct a date from a POSIX timestamp (like time.time())."
         if t is None:
             raise TypeError("'NoneType' object cannot be interpreted as an integer")
+        if t < 0 and sys.platform.startswith("win"):
+            # Windows converters throw an OSError for negative values.
+            y, m, d, hh, mm, ss, weekday, jday, dst = _time.localtime(0)
+            result = cls(y, m, d)
+            return result + timedelta(seconds=t)
         y, m, d, hh, mm, ss, weekday, jday, dst = _time.localtime(t)
         return cls(y, m, d)
 
@@ -1864,6 +1869,11 @@ def _fromtimestamp(cls, t, utc, tz):
             us += 1000000
 
         converter = _time.gmtime if utc else _time.localtime
+        if t < 0 and sys.platform.startswith("win"):
+            # Windows converters throw an OSError for negative values.
+            y, m, d, hh, mm, ss, weekday, jday, dst = converter(0)
+            result = cls(y, m, d, hh, mm, ss, us, tz)
+            return result + timedelta(seconds=t, microseconds=us)
         y, m, d, hh, mm, ss, weekday, jday, dst = converter(t)
         ss = min(ss, 59)    # clamp out leap seconds if the platform has them
         result = cls(y, m, d, hh, mm, ss, us, tz)

Lib/test/datetimetester.py

@@ -2683,24 +2683,19 @@ def utcfromtimestamp(*args, **kwargs):
             self.assertEqual(zero.second, 0)
             self.assertEqual(zero.microsecond, 0)
             one = fts(1e-6)
-            try:
-                minus_one = fts(-1e-6)
-            except OSError:
-                # localtime(-1) and gmtime(-1) is not supported on Windows
-                pass
-            else:
-                self.assertEqual(minus_one.second, 59)
-                self.assertEqual(minus_one.microsecond, 999999)
-
-                t = fts(-1e-8)
-                self.assertEqual(t, zero)
-                t = fts(-9e-7)
-                self.assertEqual(t, minus_one)
-                t = fts(-1e-7)
-                self.assertEqual(t, zero)
-                t = fts(-1/2**7)
-                self.assertEqual(t.second, 59)
-                self.assertEqual(t.microsecond, 992188)
+            minus_one = fts(-1e-6)
+            self.assertEqual(minus_one.second, 59)
+            self.assertEqual(minus_one.microsecond, 999999)
+
+            t = fts(-1e-8)
+            self.assertEqual(t, zero)
+            t = fts(-9e-7)
+            self.assertEqual(t, minus_one)
+            t = fts(-1e-7)
+            self.assertEqual(t, zero)
+            t = fts(-1/2**7)
+            self.assertEqual(t.second, 59)
+            self.assertEqual(t.microsecond, 992188)
 
             t = fts(1e-7)
             self.assertEqual(t, zero)
@@ -2835,13 +2830,11 @@ def test_insane_utcfromtimestamp(self):
                 self.assertRaises(OverflowError, self.theclass.utcfromtimestamp,
                                   insane)
 
-    @unittest.skipIf(sys.platform == "win32", "Windows doesn't accept negative timestamps")
     def test_negative_float_fromtimestamp(self):
         # The result is tz-dependent; at least test that this doesn't
         # fail (like it did before bug 1646728 was fixed).
         self.theclass.fromtimestamp(-1.05)
 
-    @unittest.skipIf(sys.platform == "win32", "Windows doesn't accept negative timestamps")
     def test_negative_float_utcfromtimestamp(self):
         with self.assertWarns(DeprecationWarning):
             d = self.theclass.utcfromtimestamp(-1.05)

Misc/NEWS.d/next/Windows/2025-05-21-12-29-59.gh-issue-80620.WKel4J.rst

@@ -0,0 +1,4 @@
+Support negative values in :meth:`datetime.date.fromtimestamp` and
+:meth:`datetime.datetime.fromtimestamp` on Windows, by substituting 0 and
+using :class:`datetime.timedelta` to go back in time. Patch by John Keith
+Hohm.

Modules/_datetimemodule.c

@@ -3250,6 +3250,20 @@ date_fromtimestamp(PyObject *cls, PyObject *obj)
     if (_PyTime_ObjectToTime_t(obj, &t, _PyTime_ROUND_FLOOR) == -1)
         return NULL;
 
+#ifdef MS_WINDOWS
+    if (t < 0) {
+	if (_PyTime_localtime(0, &tm) != 0)
+	    return NULL;
+
+	PyObject *date = new_date_subclass_ex(tm.tm_year + 1900,
+					      tm.tm_mon + 1,
+					      tm.tm_mday,
+					      cls);
+	PyObject *delta = new_delta(0, t, 0, 1);
+	return add_datetime_timedelta(date, delta, 1);
+    }
+#endif
+
     if (_PyTime_localtime(t, &tm) != 0)
         return NULL;
 
@@ -5529,6 +5543,20 @@ datetime_from_timestamp(PyObject *cls, TM_FUNC f, PyObject *timestamp,
     time_t timet;
     long us;
 
+#ifdef MS_WINDOWS
+    if (PyFloat_Check(timestamp)) {
+	if (PyFloat_AsDouble(timestamp) < 0) {
+	    if (_PyTime_ObjectToTimeval(timestamp,
+					&timet, &us, _PyTime_ROUND_HALF_EVEN) == -1)
+		return NULL;
+
+	    PyObject *dt = datetime_from_timet_and_us(cls, f, 0, 0, tzinfo);
+	    PyObject *delta = new_delta(0, timet, us, 1);
+	    return add_datetime_timedelta(date, delta, 1);
+	}
+    }
+#endif
+
     if (_PyTime_ObjectToTimeval(timestamp,
                                 &timet, &us, _PyTime_ROUND_HALF_EVEN) == -1)
         return NULL;