diff --git a/smartsim/experiment.py b/smartsim/experiment.py
index 442874f91..f525e8f85 100644
--- a/smartsim/experiment.py
+++ b/smartsim/experiment.py
@@ -182,7 +182,9 @@ def start(self, *jobs: Job | t.Sequence[Job]) -> tuple[LaunchedJobID, ...]:
         # Create the run id
         jobs_ = list(_helpers.unpack(jobs))
 
-        run_id = datetime.datetime.now().replace(microsecond=0).isoformat()
+        run_id = (
+            datetime.datetime.now().replace(microsecond=0).isoformat().replace(":", ".")
+        )
         root = pathlib.Path(self.exp_path, run_id)
         return self._dispatch(Generator(root), dispatch.DEFAULT_DISPATCHER, *jobs_)