diff --git a/swh/model/hypothesis_strategies.py b/swh/model/hypothesis_strategies.py
--- a/swh/model/hypothesis_strategies.py
+++ b/swh/model/hypothesis_strategies.py
@@ -133,7 +133,7 @@
     ).timestamp()
     defaults = dict(
         seconds=integers(min_seconds, max_seconds),
-        microseconds=integers(0, 1000000),
+        microseconds=integers(0, 1000000 - 1),
     )
     return builds(dict, **{**defaults, **kwargs})