diff --git a/swh/model/model.py b/swh/model/model.py
--- a/swh/model/model.py
+++ b/swh/model/model.py
@@ -165,7 +165,7 @@
     """Represents a TZ-aware timestamp from a VCS."""
     timestamp = attrib_typecheck(type=Timestamp)
     offset = attrib_typecheck(type=int)
-    negative_utc = attrib_typecheck(type=bool)
+    negative_utc = attrib_typecheck(type=bool, default=False)
 
     @offset.validator
     def check_offset(self, attribute, value):
@@ -176,6 +176,11 @@
             # you'll find in the wild...
             raise ValueError('offset too large: %d minutes' % value)
 
+    @negative_utc.validator
+    def check_sign(self, attribute, value):
+        if self.offset and value:
+            raise ValueError("negative_utc can only be True is offset=0")
+
     @classmethod
     def from_dict(cls, obj: Union[Dict, datetime.datetime, int]):
         """Builds a TimestampWithTimezone from any of the formats
diff --git a/swh/model/tests/test_model.py b/swh/model/tests/test_model.py
--- a/swh/model/tests/test_model.py
+++ b/swh/model/tests/test_model.py
@@ -90,6 +90,63 @@
         Timestamp(seconds=0, microseconds=-1)
 
 
+def test_timestampwithtimezone():
+    ts = Timestamp(seconds=0, microseconds=0)
+    tstz = TimestampWithTimezone(
+        timestamp=ts,
+        offset=0)
+    attr.validate(tstz)
+    assert tstz.negative_utc is False
+
+    attr.validate(TimestampWithTimezone(
+        timestamp=ts,
+        offset=10))
+
+    attr.validate(TimestampWithTimezone(
+        timestamp=ts,
+        offset=-10))
+
+    tstz = TimestampWithTimezone(
+        timestamp=ts,
+        offset=0,
+        negative_utc=True)
+    attr.validate(tstz)
+    assert tstz.negative_utc is True
+
+    with pytest.raises(AttributeTypeError):
+        TimestampWithTimezone(
+            timestamp=datetime.datetime.now(),
+            offset=0)
+
+    with pytest.raises(AttributeTypeError):
+        TimestampWithTimezone(
+            timestamp=ts,
+            offset='0')
+
+    with pytest.raises(AttributeTypeError):
+        TimestampWithTimezone(
+            timestamp=ts,
+            offset=1.0)
+
+    with pytest.raises(AttributeTypeError):
+        TimestampWithTimezone(
+            timestamp=ts,
+            offset=1,
+            negative_utc=0)
+
+    with pytest.raises(ValueError):
+        TimestampWithTimezone(
+            timestamp=ts,
+            offset=1,
+            negative_utc=True)
+
+    with pytest.raises(ValueError):
+        TimestampWithTimezone(
+            timestamp=ts,
+            offset=-1,
+            negative_utc=True)
+
+
 def test_timestampwithtimezone_from_datetime():
     tz = datetime.timezone(datetime.timedelta(minutes=+60))
     date = datetime.datetime(