diff --git a/tests/dns_tests/test_service_statistics.py b/tests/dns_tests/test_service_statistics.py
index 0088fcd6..0d09a368 100644
--- a/tests/dns_tests/test_service_statistics.py
+++ b/tests/dns_tests/test_service_statistics.py
@@ -50,7 +50,9 @@ async def test_queries_leave_no_per_profile_document(self, user, mongo_db):
         settings: no document references the profile; service_statistics gains
         anonymous, PoP-labelled hourly counters in a single document."""
         pid = user.new_profile("service-stats")
-        hour_start = _utc(time.time() - time.time() % HOUR_S)
+        # One clock read: two reads straddle a sub-microsecond gap and round below the hour.
+        now = time.time()
+        hour_start = _utc(now - now % HOUR_S)
         since = hour_start
         baseline = _service_total_since(mongo_db, since)