Fix timestamp parsing (#415)

Fixes #407

tests/inputs/google_impl_behavior_equivalence/google_impl_behavior_equivalence.proto

@@ -1,5 +1,6 @@
 syntax = "proto3";
 
+import "google/protobuf/timestamp.proto";
 package google_impl_behavior_equivalence;
 
 message Foo { int64 bar = 1; }
@@ -12,6 +13,10 @@ message Test {
   }
 }
 
+message Spam {
+  google.protobuf.Timestamp ts = 1;
+}
+
 message Request { Empty foo = 1; }
 
-message Empty {}
+message Empty {}

tests/inputs/google_impl_behavior_equivalence/test_google_impl_behavior_equivalence.py

@@ -1,17 +1,25 @@
+from datetime import (
+    datetime,
+    timezone,
+)
+
 import pytest
 from google.protobuf import json_format
+from google.protobuf.timestamp_pb2 import Timestamp
 
 import betterproto
 from tests.output_betterproto.google_impl_behavior_equivalence import (
     Empty,
     Foo,
     Request,
+    Spam,
     Test,
 )
 from tests.output_reference.google_impl_behavior_equivalence.google_impl_behavior_equivalence_pb2 import (
     Empty as ReferenceEmpty,
     Foo as ReferenceFoo,
     Request as ReferenceRequest,
+    Spam as ReferenceSpam,
     Test as ReferenceTest,
 )
 
@@ -59,6 +67,19 @@ def test_bytes_are_the_same_for_oneof():
     assert isinstance(message_reference2.foo, ReferenceFoo)
 
 
+@pytest.mark.parametrize("dt", (datetime.min.replace(tzinfo=timezone.utc),))
+def test_datetime_clamping(dt):  # see #407
+    ts = Timestamp()
+    ts.FromDatetime(dt)
+    assert bytes(Spam(dt)) == ReferenceSpam(ts=ts).SerializeToString()
+    message_bytes = bytes(Spam(dt))
+
+    assert (
+        Spam().parse(message_bytes).ts.timestamp()
+        == ReferenceSpam.FromString(message_bytes).ts.seconds
+    )
+
+
 def test_empty_message_field():
     message = Request()
     reference_message = ReferenceRequest()