diff --git a/swh/model/model.py b/swh/model/model.py
--- a/swh/model/model.py
+++ b/swh/model/model.py
@@ -456,10 +456,19 @@
         offset_str = offset_bytes.decode()
         assert offset_str[0] in "+-"
         sign = int(offset_str[0] + "1")
-        hours = int(offset_str[1:-2])
-        minutes = int(offset_str[-2:])
+        if len(offset_str) <= 3:
+            hours = int(offset_str[1:])
+            minutes = 0
+        else:
+            hours = int(offset_str[1:-2])
+            minutes = int(offset_str[-2:])
+
         offset = sign * (hours * 60 + minutes)
-        return offset
+        if (0 <= minutes <= 59) and (-(2 ** 15) <= offset < 2 ** 15):
+            return offset
+        else:
+            # can't parse it to a reasonable value; give up and pretend it's UTC.
+            return 0
 
     def _check_offsets_match(self):
         offset = self._parse_offset_bytes(self.offset_bytes)
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
@@ -480,6 +480,21 @@
     assert tstz.offset == 800
     assert tstz.offset_bytes == b"+1320"
 
+    tstz = TimestampWithTimezone(timestamp=ts, offset_bytes=b"+200")
+    attr.validate(tstz)
+    assert tstz.offset == 120
+    assert tstz.offset_bytes == b"+200"
+
+    tstz = TimestampWithTimezone(timestamp=ts, offset_bytes=b"+02")
+    attr.validate(tstz)
+    assert tstz.offset == 120
+    assert tstz.offset_bytes == b"+02"
+
+    tstz = TimestampWithTimezone(timestamp=ts, offset_bytes=b"+2000000000")
+    attr.validate(tstz)
+    assert tstz.offset == 0
+    assert tstz.offset_bytes == b"+2000000000"
+
     with pytest.raises(AttributeTypeError):
         TimestampWithTimezone(timestamp=datetime.datetime.now(), offset_bytes=b"+0000")