fix: datetime before epoch on windows in cython implementation

Cython implementation still used datetime.from_timestamp method, which does not work on windows.
Update the cython implementation to use utc time and delta
and add a regression test to highlight the issue.

Author: peterfischer

msgpack/unpack.h

@@ -342,32 +342,53 @@ static int unpack_callback_ext(unpack_user* u, const char* base, const char* pos
             py = PyObject_CallFunction(u->timestamp_t, "(Lk)", ts.tv_sec, ts.tv_nsec);
         }
         else { // float or datetime
-            PyObject *a = PyFloat_FromDouble((double)ts.tv_nsec);
-            if (a == NULL) return -1;
+            if (u->timestamp == 3) {  // datetime
+                // Calculate datetime using epoch + delta 
+                // due to limitations PyDateTime_FromTimestamp on Windows with negative timestamps
+                PyObject *t0 = PyLong_FromLongLong(0L);
+                if (t0 == NULL) {
+                    return -1;
+                }
 
-            PyObject *b = PyNumber_TrueDivide(a, u->giga);
-            Py_DECREF(a);
-            if (b == NULL) return -1;
+                PyObject *t = PyTuple_Pack(2, t0, u->utc);
+                Py_DECREF(t0);
+                if (t == NULL) {
+                    return -1;
+                }
 
-            PyObject *c = PyLong_FromLongLong(ts.tv_sec);
-            if (c == NULL) {
-                Py_DECREF(b);
-                return -1;
-            }
+                PyObject *t_epoch = PyDateTime_FromTimestamp(t);
+                Py_DECREF(t);
+                if (t_epoch == NULL) {
+                    return -1;
+                }
 
-            a = PyNumber_Add(b, c);
-            Py_DECREF(b);
-            Py_DECREF(c);
+                PyObject* d = PyDelta_FromDSU(0, ts.tv_sec, ts.tv_nsec / 1000);
+                if (d == NULL) {
+                    Py_DECREF(t_epoch);
+                    return -1;
+                }
 
-            if (u->timestamp == 3) {  // datetime
-                PyObject *t = PyTuple_Pack(2, a, u->utc);
+                py = PyObject_CallMethod(t_epoch, "__add__", "(O)", d);
+                
+                Py_DECREF(t_epoch);
+                Py_DECREF(d);
+            } else { // float
+                PyObject *a = PyFloat_FromDouble((double)ts.tv_nsec);
+                if (a == NULL) return -1;
+
+                PyObject *b = PyNumber_TrueDivide(a, u->giga);
                 Py_DECREF(a);
-                if (t == NULL) {
+                if (b == NULL) return -1;
+
+                PyObject *c = PyLong_FromLongLong(ts.tv_sec);
+                if (c == NULL) {
+                    Py_DECREF(b);
                     return -1;
                 }
-                py = PyDateTime_FromTimestamp(t);
-                Py_DECREF(t);
-            } else { // float
+
+                a = PyNumber_Add(b, c);
+                Py_DECREF(b);
+                Py_DECREF(c);
                 py = a;
             }
         }

test/test_timestamp.py

@@ -99,6 +99,14 @@ def test_unpack_datetime():
     assert unpacked == datetime.datetime(1970, 1, 1, 0, 0, 42, 0, tzinfo=_utc)
 
 
+@pytest.mark.skipif(sys.version_info[0] == 2, reason="datetime support is PY3+ only")
+def test_pack_unpack_before_epoch():
+    t_in = datetime.datetime(1960, 1, 1, tzinfo=_utc)
+    packed = msgpack.packb(t_in, datetime=True)
+    unpacked = msgpack.unpackb(packed, timestamp=3)
+    assert unpacked == t_in
+
+
 @pytest.mark.skipif(sys.version_info[0] == 2, reason="datetime support is PY3+ only")
 def test_pack_datetime():
     t = Timestamp(42, 14000)