diff --git a/swh/storage/in_memory.py b/swh/storage/in_memory.py
--- a/swh/storage/in_memory.py
+++ b/swh/storage/in_memory.py
@@ -1250,6 +1250,11 @@
                 snapshot (sha1_git): identifier of the snapshot to add to
                     the visit
         """
+        visits = copy.deepcopy(visits)
+        for visit in visits:
+            if isinstance(visit['date'], str):
+                visit['date'] = dateutil.parser.parse(visit['date'])
+
         if self.journal_writer:
             for visit in visits:
                 visit = visit.copy()
@@ -1260,8 +1265,6 @@
         for visit in visits:
             origin_id = visit['origin']
             visit_id = visit['visit']
-            if isinstance(visit['date'], str):
-                visit['date'] = dateutil.parser.parse(visit['date'])
             while len(self._origin_visits[origin_id-1]) < visit_id:
                 self._origin_visits[origin_id-1].append(None)
             visit = self._origin_visits[origin_id-1][visit_id-1] = visit
diff --git a/swh/storage/storage.py b/swh/storage/storage.py
--- a/swh/storage/storage.py
+++ b/swh/storage/storage.py
@@ -1257,6 +1257,11 @@
                 snapshot (sha1_git): identifier of the snapshot to add to
                     the visit
         """
+        visits = copy.deepcopy(visits)
+        for visit in visits:
+            if isinstance(visit['date'], str):
+                visit['date'] = dateutil.parser.parse(visit['date'])
+
         if self.journal_writer:
             for visit in visits:
                 visit = visit.copy()
diff --git a/swh/storage/tests/test_storage.py b/swh/storage/tests/test_storage.py
--- a/swh/storage/tests/test_storage.py
+++ b/swh/storage/tests/test_storage.py
@@ -1664,29 +1664,48 @@
         self.assertIsNotNone(origin_id)
 
         # when
-        self.storage.origin_visit_upsert([{
-             'origin': origin_id,
-             'date': self.date_visit2,
-             'visit': 123,
-             'status': 'full',
-             'metadata': None,
-             'snapshot': None,
-         }])
+        self.storage.origin_visit_upsert([
+            {
+                 'origin': origin_id,
+                 'date': self.date_visit2,
+                 'visit': 123,
+                 'status': 'full',
+                 'metadata': None,
+                 'snapshot': None,
+             },
+            {
+                 'origin': origin_id,
+                 'date': '2018-01-01 23:00:00+00',
+                 'visit': 1234,
+                 'status': 'full',
+                 'metadata': None,
+                 'snapshot': None,
+             },
+        ])
 
         # then
         actual_origin_visits = list(self.storage.origin_visit_get(origin_id))
-        self.assertEqual(actual_origin_visits,
-                         [{
-                             'origin': origin_id,
-                             'date': self.date_visit2,
-                             'visit': 123,
-                             'status': 'full',
-                             'metadata': None,
-                             'snapshot': None,
-                         }])
+        self.assertEqual(actual_origin_visits, [
+            {
+                'origin': origin_id,
+                'date': self.date_visit2,
+                'visit': 123,
+                'status': 'full',
+                'metadata': None,
+                'snapshot': None,
+            },
+            {
+                'origin': origin_id,
+                'date': self.date_visit3,
+                'visit': 1234,
+                'status': 'full',
+                'metadata': None,
+                'snapshot': None,
+            },
+        ])
 
         expected_origin = self.origin2.copy()
-        data = {
+        data1 = {
             'origin': expected_origin,
             'date': self.date_visit2,
             'visit': 123,
@@ -1694,9 +1713,18 @@
             'metadata': None,
             'snapshot': None,
         }
+        data2 = {
+            'origin': expected_origin,
+            'date': self.date_visit3,
+            'visit': 1234,
+            'status': 'full',
+            'metadata': None,
+            'snapshot': None,
+        }
         self.assertEqual(list(self.journal_writer.objects),
                          [('origin', expected_origin),
-                          ('origin_visit', data)])
+                          ('origin_visit', data1),
+                          ('origin_visit', data2)])
 
     def test_origin_visit_upsert_existing(self):
         # given