diff --git a/swh/storage/tests/storage_data.py b/swh/storage/tests/storage_data.py
--- a/swh/storage/tests/storage_data.py
+++ b/swh/storage/tests/storage_data.py
@@ -201,7 +201,7 @@
     'date': {
         'timestamp': 1234567890,
         'offset': 120,
-        'negative_utc': None,
+        'negative_utc': False,
     },
     'committer': {
         'name': b'St\xc3fano Zacchiroli',
@@ -245,7 +245,7 @@
             'microseconds': 220000,
         },
         'offset': -720,
-        'negative_utc': None,
+        'negative_utc': False,
     },
     'committer': {
         'name': b'tony',
@@ -279,7 +279,7 @@
             'microseconds': 220000,
         },
         'offset': -720,
-        'negative_utc': None,
+        'negative_utc': False,
     },
     'committer': {
         'name': b'tony',
@@ -313,7 +313,7 @@
             'microseconds': 220000,
         },
         'offset': -720,
-        'negative_utc': None,
+        'negative_utc': False,
     },
     'committer': {
         'name': b'committer-dude',
@@ -326,7 +326,7 @@
             'microseconds': 220000,
         },
         'offset': -720,
-        'negative_utc': None,
+        'negative_utc': False,
     },
     'parents': [hash_to_bytes(  # revision3
         '7026b7c1a2af56521e951c01ed20f255fa054238')],
@@ -391,7 +391,7 @@
     'date': {
         'timestamp': 1234567890,
         'offset': 42,
-        'negative_utc': None,
+        'negative_utc': False,
     },
     'target': b'43210987654321098765',
     'target_type': 'revision',
@@ -410,7 +410,7 @@
     'date': {
         'timestamp': 1634366813,
         'offset': -120,
-        'negative_utc': None,
+        'negative_utc': False,
     },
     'target': b'432109\xa9765432\xc309\x00765',
     'target_type': 'revision',
diff --git a/swh/storage/tests/test_retry.py b/swh/storage/tests/test_retry.py
--- a/swh/storage/tests/test_retry.py
+++ b/swh/storage/tests/test_retry.py
@@ -3,6 +3,7 @@
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
+from datetime import datetime
 from typing import Dict
 from unittest.mock import call
 
@@ -266,7 +267,10 @@
     origin = list(swh_storage.origin_visit_get(origin_url))
     assert not origin
 
-    origin_visit = swh_storage.origin_visit_add(origin_url, '2020-01-01', 'hg')
+    origin_visit = swh_storage.origin_visit_add(
+        origin_url,
+        datetime(2020, 1, 1),
+        'hg')
     assert origin_visit.origin == origin_url
     assert isinstance(origin_visit.visit, int)
 
@@ -300,13 +304,16 @@
     origin = list(swh_storage.origin_visit_get(origin_url))
     assert not origin
 
-    r = swh_storage.origin_visit_add(origin_url, '2020-01-01', 'git')
+    r = swh_storage.origin_visit_add(
+        origin_url,
+        datetime(2020, 1, 1),
+        'git')
     assert r == {'origin': origin_url, 'visit': 1}
 
     mock_memory.assert_has_calls([
-        call(origin_url, '2020-01-01', 'git'),
-        call(origin_url, '2020-01-01', 'git'),
-        call(origin_url, '2020-01-01', 'git')
+        call(origin_url, datetime(2020, 1, 1), 'git'),
+        call(origin_url, datetime(2020, 1, 1), 'git'),
+        call(origin_url, datetime(2020, 1, 1), 'git')
     ])
     assert mock_sleep.call_count == 2
 
@@ -327,10 +334,15 @@
     assert not origin
 
     with pytest.raises(StorageArgumentException, match='Refuse to add'):
-        swh_storage.origin_visit_add(origin_url, '2020-01-31', 'svn')
+        swh_storage.origin_visit_add(
+            origin_url,
+            datetime(2020, 1, 31),
+            'svn')
 
     mock_memory.assert_has_calls([
-        call(origin_url, '2020-01-31', 'svn'),
+        call(origin_url,
+             datetime(2020, 1, 31),
+             'svn'),
     ])
 
 
@@ -599,7 +611,10 @@
     """
     sample_origin = sample_data['origin'][0]
     origin_url = swh_storage.origin_add_one(sample_origin)
-    origin_visit = swh_storage.origin_visit_add(origin_url, '2020-01-01', 'hg')
+    origin_visit = swh_storage.origin_visit_add(
+        origin_url,
+        datetime(2020, 1, 1),
+        'hg')
 
     ov = next(swh_storage.origin_visit_get(origin_url))
     assert ov['origin'] == origin_url