Fix datetime before epoch on windows in cython implementation (#436)

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.

Makefile

@@ -25,6 +25,7 @@ clean:
 	rm -rf build
 	rm -f msgpack/_cmsgpack.cpp
 	rm -f msgpack/_cmsgpack.*.so
+	rm -f msgpack/_cmsgpack.*.pyd
 	rm -rf msgpack/__pycache__
 	rm -rf test/__pycache__
 

msgpack/unpack.h

@@ -341,7 +341,26 @@ static int unpack_callback_ext(unpack_user* u, const char* base, const char* pos
         else if (u->timestamp == 0) {  // Timestamp
             py = PyObject_CallFunction(u->timestamp_t, "(Lk)", ts.tv_sec, ts.tv_nsec);
         }
-        else { // float or datetime
+        else if (u->timestamp == 3) {  // datetime
+            // Calculate datetime using epoch + delta 
+            // due to limitations PyDateTime_FromTimestamp on Windows with negative timestamps
+            PyObject *epoch = PyDateTimeAPI->DateTime_FromDateAndTime(1970, 1, 1, 0, 0, 0, 0, u->utc, PyDateTimeAPI->DateTimeType);
+            if (epoch == NULL) {
+                return -1;
+            }
+
+            PyObject* d = PyDelta_FromDSU(0, ts.tv_sec, ts.tv_nsec / 1000);
+            if (d == NULL) {
+                Py_DECREF(epoch);
+                return -1;
+            }
+
+            py = PyNumber_Add(epoch, d);
+            
+            Py_DECREF(epoch);
+            Py_DECREF(d);
+        }
+        else { // float
             PyObject *a = PyFloat_FromDouble((double)ts.tv_nsec);
             if (a == NULL) return -1;
 
@@ -358,18 +377,7 @@ static int unpack_callback_ext(unpack_user* u, const char* base, const char* pos
             a = PyNumber_Add(b, c);
             Py_DECREF(b);
             Py_DECREF(c);
-
-            if (u->timestamp == 3) {  // datetime
-                PyObject *t = PyTuple_Pack(2, a, u->utc);
-                Py_DECREF(a);
-                if (t == NULL) {
-                    return -1;
-                }
-                py = PyDateTime_FromTimestamp(t);
-                Py_DECREF(t);
-            } else { // float
-                py = a;
-            }
+            py = a;
         }
     } else {
         py = PyObject_CallFunction(u->ext_hook, "(iy#)", (int)typecode, pos, (Py_ssize_t)length-1);

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)