modified   swh/model/model.py
@@ -175,7 +175,7 @@ class TimestampWithTimezone(BaseModel):
 
     timestamp = attr.ib(type=Timestamp, validator=type_validator())
     offset = attr.ib(type=int, validator=type_validator())
-    negative_utc = attr.ib(type=bool, validator=type_validator())
+    negative_utc = attr.ib(type=Optional[bool], validator=type_validator())
 
     @offset.validator
     def check_offset(self, attribute, value):
@@ -188,8 +188,10 @@ class TimestampWithTimezone(BaseModel):
 
     @negative_utc.validator
     def check_negative_utc(self, attribute, value):
+        if value is None and self.offset == 0:
+            raise ValueError("negative_utc can only be unset when offset != 0")
         if self.offset and value:
-            raise ValueError("negative_utc can only be True is offset=0")
+            raise ValueError("negative_utc can only be True when offset == 0")
 
     @classmethod
     def from_dict(cls, obj: Union[Dict, datetime.datetime, int]):