diff --git a/src/datasets/packaged_modules/json/json.py b/src/datasets/packaged_modules/json/json.py
index 9845dd676d4..b49aecf197f 100644
--- a/src/datasets/packaged_modules/json/json.py
+++ b/src/datasets/packaged_modules/json/json.py
@@ -181,9 +181,9 @@ def _generate_tables(self, base_files, files_iterables, original_files, allow_fu
                     for i, training_example in enumerate(training_examples):
                         if training_example["metadata"]["trace_type"] == "hermes":
                             timestamp = ujson_loads(lines[i])["started_at"]
-                            milliseconds = timestamp if timestamp <= 10_000_000_000 else timestamp * 1_000
+                            seconds = timestamp if timestamp <= 10_000_000_000 else timestamp / 1_000
                             sent_at = (
-                                datetime.fromtimestamp(milliseconds, tz=timezone.utc)
+                                datetime.fromtimestamp(seconds, tz=timezone.utc)
                                 .isoformat(timespec="milliseconds")
                                 .replace("+00:00", "Z")
                             )
diff --git a/tests/packaged_modules/test_json.py b/tests/packaged_modules/test_json.py
index 2d6fd5b3849..556c192817d 100644
--- a/tests/packaged_modules/test_json.py
+++ b/tests/packaged_modules/test_json.py
@@ -447,6 +447,10 @@ def assert_agent_traces_output(tmp_path, filename, rows, expected, num_sessions=
     ],
 }
 
+# Same session, but `started_at` expressed in milliseconds (e.g. JS `Date.now()`-style)
+# instead of seconds -- the same real-world instant as HERMES_SESSION.
+HERMES_SESSION_MS = {**HERMES_SESSION, "started_at": 1_780_665_768_307}
+
 
 DROID_SESSION = [
     {
@@ -721,6 +725,14 @@ def test_json_generate_tables_with_sorted_columns(file_fixture, config_kwargs, r
             [HERMES_SESSION] * 2,
             ("hermes", "20260605_092247_d018ec", "Run pwd and date.", "2026-06-05T13:22:48.307Z", 1, 1),
         ),
+        pytest.param(
+            "hermes_ms.jsonl",
+            [HERMES_SESSION_MS],
+            # Same instant as the seconds-form "hermes" case above -- `started_at` is
+            # just expressed in milliseconds here.
+            ("hermes", "20260605_092247_d018ec", "Run pwd and date.", "2026-06-05T13:22:48.307Z", 1, 1),
+            id="hermes-ms",
+        ),
         pytest.param(
             "droid.jsonl",
             DROID_SESSION,