diff --git a/scripts/lib/skill-evolution/tracker.js b/scripts/lib/skill-evolution/tracker.js
index f243cf802..da8005aed 100644
--- a/scripts/lib/skill-evolution/tracker.js
+++ b/scripts/lib/skill-evolution/tracker.js
@@ -79,7 +79,7 @@ function normalizeExecutionRecord(input, options = {}) {
     throw new Error('failure_reason must be a string or null');
   }
 
-  if (Number.isNaN(Date.parse(recordedAt))) {
+  if (typeof recordedAt !== 'string' || Number.isNaN(Date.parse(recordedAt))) {
     throw new Error('recorded_at must be an ISO timestamp');
   }
 
diff --git a/tests/lib/skill-tracker-timestamps.test.js b/tests/lib/skill-tracker-timestamps.test.js
new file mode 100644
index 000000000..dad09762b
--- /dev/null
+++ b/tests/lib/skill-tracker-timestamps.test.js
@@ -0,0 +1,91 @@
+'use strict';
+
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const os = require('node:os');
+const path = require('node:path');
+const { test } = require('node:test');
+const tracker = require('../../scripts/lib/skill-evolution/tracker');
+
+const execution = {
+  skill_id: 'demo',
+  skill_version: 'v1',
+  task_description: 'Run a temporary task',
+  outcome: 'success',
+};
+const timestamp = '2026-03-15T12:32:00.000Z';
+const invalidTimestamps = [2026, [timestamp], new Date(timestamp), {}];
+
+function withRunsFile(fn) {
+  const root = fs.mkdtempSync(path.join(os.tmpdir(), 'skill-tracker-timestamps-'));
+  const runsFilePath = path.join(root, 'runs.jsonl');
+  try {
+    fn({ runsFilePath });
+  } finally {
+    fs.rmSync(root, { recursive: true, force: true });
+  }
+}
+
+test('rejects non-string execution timestamps before creating storage', () => {
+  withRunsFile(options => {
+    for (const recorded_at of invalidTimestamps) {
+      assert.throws(
+        () => tracker.recordSkillExecution({ ...execution, recorded_at }, options),
+        /recorded_at must be an ISO timestamp/
+      );
+      assert.equal(fs.existsSync(options.runsFilePath), false);
+    }
+  });
+});
+
+test('rejects invalid clock overrides without changing existing records', () => {
+  withRunsFile(options => {
+    tracker.recordSkillExecution({ ...execution, recorded_at: timestamp }, options);
+    const previousBytes = fs.readFileSync(options.runsFilePath, 'utf8');
+    for (const now of invalidTimestamps) {
+      assert.throws(
+        () => tracker.recordSkillExecution(execution, { ...options, now }),
+        /recorded_at must be an ISO timestamp/
+      );
+      assert.equal(fs.readFileSync(options.runsFilePath, 'utf8'), previousBytes);
+    }
+    assert.equal(tracker.readSkillExecutionRecords(options).length, 1);
+  });
+});
+
+test('retains explicit and clock-supplied string timestamps on readback', () => {
+  withRunsFile(options => {
+    const offsetTimestamp = '2026-03-15T14:32:00+02:00';
+    tracker.recordSkillExecution({ ...execution, recorded_at: offsetTimestamp }, { ...options, now: 2026 });
+    tracker.recordSkillExecution(execution, { ...options, now: timestamp });
+    assert.deepEqual(
+      tracker.readSkillExecutionRecords(options).map(record => record.recorded_at),
+      [offsetTimestamp, timestamp]
+    );
+    tracker.recordSkillExecution(execution, options);
+    const records = tracker.readSkillExecutionRecords(options);
+    assert.equal(records.length, 3);
+    assert.equal(typeof records[2].recorded_at, 'string');
+    assert.ok(Number.isFinite(Date.parse(records[2].recorded_at)));
+  });
+});
+
+test('rejects invalid timestamps before submitting to a state store', () => {
+  withRunsFile(options => {
+    let writes = 0;
+    const stateStore = {
+      recordSkillExecution() {
+        writes += 1;
+        return {};
+      },
+    };
+    for (const recorded_at of invalidTimestamps) {
+      assert.throws(
+        () => tracker.recordSkillExecution({ ...execution, recorded_at }, { ...options, stateStore }),
+        /recorded_at must be an ISO timestamp/
+      );
+      assert.equal(writes, 0);
+      assert.equal(fs.existsSync(options.runsFilePath), false);
+    }
+  });
+});