fix: _Timestamp.to_datetime works with negative ts

src/betterproto/__init__.py

@@ -1578,6 +1578,10 @@ class _Timestamp(Timestamp):
 
     def to_datetime(self) -> datetime:
         ts = self.seconds + (self.nanos / 1e9)
+
+        if ts < 0:
+            return datetime(1970, 1, 1) + timedelta(seconds=ts)
+        else:
             return datetime.fromtimestamp(ts, tz=timezone.utc)
 
     @staticmethod