gh-81708: Support negative datetime.datetime.timestamp values from naive datetimes on Windows

Lib/_pydatetime.py

@@ -8,7 +8,7 @@
 
 import time as _time
 import math as _math
-import sys
+import os
 from operator import index as _index
 
 def _cmp(x, y):
@@ -1877,7 +1877,7 @@ def _fromtimestamp(cls, t, utc, tz):
             # thus we can't perform fold detection for values of time less
             # than the max time fold. See comments in _datetimemodule's
             # version of this method for more details.
-            if t < max_fold_seconds and sys.platform.startswith("win"):
+            if t < max_fold_seconds and os.name == 'nt':
                 return result
 
             y, m, d, hh, mm, ss = converter(t - max_fold_seconds)[:6]
@@ -2050,6 +2050,9 @@ def local(u):
     def timestamp(self):
         "Return POSIX timestamp as float"
         if self._tzinfo is None:
+            if self < _NAIVE_EPOCH and os.name == 'nt':
+                # Windows converters throw an OSError for negative values.
+                return (self - _NAIVE_EPOCH).total_seconds()
             s = self._mktime()
             return s + self.microsecond / 1e6
         else:
@@ -2561,6 +2564,7 @@ def _name_from_offset(delta):
 timezone.min = timezone._create(-timedelta(hours=23, minutes=59))
 timezone.max = timezone._create(timedelta(hours=23, minutes=59))
 _EPOCH = datetime(1970, 1, 1, tzinfo=timezone.utc)
+_NAIVE_EPOCH = datetime(1970, 1, 1)
 
 # Some time zone algebra.  For a datetime x, let
 #     x.n = x stripped of its timezone -- its naive time.

Lib/test/datetimetester.py

@@ -2671,6 +2671,10 @@ def test_timestamp_aware(self):
         self.assertEqual(t.timestamp(),
                          18000 + 3600 + 2*60 + 3 + 4*1e-6)
 
+    def test_naive_timestamp_before_epoch(self):
+        # Before #81708 this raised OSError on Windows.
+        self.assertLess(self.theclass(1969,1,1).timestamp(), 0)
+
     @support.run_with_tz('MSK-03')  # Something east of Greenwich
     def test_microsecond_rounding(self):
         def utcfromtimestamp(*args, **kwargs):

Misc/NEWS.d/next/Windows/2025-05-22-10-33-54.gh-issue-81708.xBa0wa.rst

@@ -0,0 +1,3 @@
+Support negative :meth:`datetime.datetime.timestamp` values from naive
+datetimes (before the UNIX epoch of 1970-01-01) on Windows by
+subtracting a naive epoch. Patch by John Keith Hohm.

Modules/_datetimemodule.c

@@ -6869,6 +6869,20 @@ datetime_timestamp(PyObject *op, PyObject *Py_UNUSED(dummy))
         result = delta_total_seconds(delta, NULL);
         Py_DECREF(delta);
     }
+#ifdef MS_WINDOWS
+    else if (GET_YEAR(self) < 1970) {
+        PyObject *naive_epoch = new_datetime(1970, 1, 1, 0, 0, 0, 0, Py_None, 0);
+        if (naive_epoch == NULL) {
+            return NULL;
+        }
+        PyObject *delta = datetime_subtract(op, naive_epoch);
+        Py_DECREF(naive_epoch);
+        if (delta == NULL)
+            return NULL;
+        result = delta_total_seconds(delta, NULL);
+        Py_DECREF(delta);
+    }
+#endif
     else {
         long long seconds;
         seconds = local_to_seconds(GET_YEAR(self),