diff --git a/rq/job.py b/rq/job.py
index 8e15224..69c18e1 100644
--- a/rq/job.py
+++ b/rq/job.py
@@ -531,10 +531,9 @@ class Job(object):
             obj['description'] = self.description
         if self.enqueued_at is not None:
             obj['enqueued_at'] = utcformat(self.enqueued_at)
-        if self.started_at is not None:
-            obj['started_at'] = utcformat(self.started_at)
-        if self.ended_at is not None:
-            obj['ended_at'] = utcformat(self.ended_at)
+
+        obj['started_at'] = utcformat(self.started_at) if self.started_at else ''
+        obj['ended_at'] = utcformat(self.ended_at) if self.ended_at else ''
         if self._result is not None:
             try:
                 obj['result'] = dumps(self._result)
diff --git a/rq/registry.py b/rq/registry.py
index eb45682..fa052d5 100644
--- a/rq/registry.py
+++ b/rq/registry.py
@@ -214,11 +214,16 @@ class FailedJobRegistry(BaseRegistry):
         result = self.connection.zrem(self.key, job.id)
         if not result:
             raise InvalidJobOperation
-
-        queue = Queue(job.origin, connection=self.connection,
-                      job_class=self.job_class)
-
-        return queue.enqueue_job(job)
+        
+        with self.connection.pipeline() as pipeline:
+            queue = Queue(job.origin, connection=self.connection,
+                          job_class=self.job_class)
+            job.started_at = None
+            job.ended_at = None
+            job.save()
+            job = queue.enqueue_job(job, pipeline=pipeline)
+            pipeline.execute()
+        return job
 
 
 class DeferredJobRegistry(BaseRegistry):
diff --git a/rq/utils.py b/rq/utils.py
index 0f63125..0058ce4 100644
--- a/rq/utils.py
+++ b/rq/utils.py
@@ -226,7 +226,7 @@ def backend_class(holder, default_name, override=None):
 
 
 def str_to_date(date_str):
-    if date_str is None:
+    if not date_str:
         return
     else:
         return utcparse(as_text(date_str))
diff --git a/tests/test_job.py b/tests/test_job.py
index cd4241c..a7a1b27 100644
--- a/tests/test_job.py
+++ b/tests/test_job.py
@@ -219,7 +219,7 @@ class TestJob(RQTestCase):
         # ... and no other keys are stored
         self.assertEqual(
             sorted(self.testconn.hkeys(job.key)),
-            [b'created_at', b'data', b'description'])
+            [b'created_at', b'data', b'description', b'ended_at', b'started_at'])
 
     def test_persistence_of_parent_job(self):
         """Storing jobs with parent job, either instance or key."""
diff --git a/tests/test_registry.py b/tests/test_registry.py
index 0fb50bc..cc1f4e7 100644
--- a/tests/test_registry.py
+++ b/tests/test_registry.py
@@ -356,6 +356,8 @@ class TestFailedJobRegistry(RQTestCase):
 
         job.refresh()
         self.assertEqual(job.get_status(), JobStatus.QUEUED)
+        self.assertEqual(job.started_at, None)
+        self.assertEqual(job.ended_at, None)
 
         worker.work(burst=True)
         self.assertTrue(job in registry)