diff --git a/CHANGELOG.md b/CHANGELOG.md
index fd78e970..2fa2582d 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -5,6 +5,9 @@ Minor changes:
     so a published result can be traced back to the code that produced it. Until
     now `task_info.yaml` was a verbatim copy of `_viash.yaml`, whose `version` is
     the revision (`build_main`) rather than a commit.
+  - `run_benchmark`: write the workflow launch time into `task_info.yaml` as
+    `timestamp`. The reporting side requires one and was falling back to guessing
+    it from the results path or a file modification time.
 
 Bug fixes:
   - `downstream`: normalise the simulated dataset the same way as the real one,
diff --git a/src/workflows/run_benchmark/main.nf b/src/workflows/run_benchmark/main.nf
index b634d763..fc4e5566 100644
--- a/src/workflows/run_benchmark/main.nf
+++ b/src/workflows/run_benchmark/main.nf
@@ -171,6 +171,9 @@ workflow run_wf {
       if (workflow.commitId) {
         task_info.commit = workflow.commitId
       }
+      // the launch time -- workflow.complete is only known once the run is over
+      task_info.timestamp = workflow.start.toInstant()
+        .truncatedTo(java.time.temporal.ChronoUnit.SECONDS).toString()
       def task_info_file = tempFile("task_info.yaml")
       task_info_file.write(toYamlBlob(task_info))