diff --git a/src/borg/testsuite/archiver.py b/src/borg/testsuite/archiver.py
index 0d28eb310..0eaea5e74 100644
--- a/src/borg/testsuite/archiver.py
+++ b/src/borg/testsuite/archiver.py
@@ -60,6 +60,8 @@ from . import key
 
 src_dir = os.path.abspath(os.path.join(os.path.dirname(__file__), '..'))
 
+ISO_FORMAT = '%Y-%m-%dT%H:%M:%S'
+
 
 def exec_cmd(*args, archiver=None, fork=False, exe=None, input=b'', binary_output=False, **kw):
     if fork:
@@ -1304,6 +1306,7 @@ class ArchiverTestCase(ArchiverTestCaseBase):
         repository = info_repo['repository']
         assert len(repository['id']) == 64
         assert 'last_modified' in repository
+        assert datetime.strptime(repository['last_modified'], ISO_FORMAT)  # must not raise
         assert info_repo['encryption']['mode'] == 'repokey'
         assert 'keyfile' not in info_repo['encryption']
         cache = info_repo['cache']
@@ -1846,9 +1849,11 @@ class ArchiverTestCase(ArchiverTestCaseBase):
         list_repo = json.loads(self.cmd('list', '--json', self.repository_location))
         repository = list_repo['repository']
         assert len(repository['id']) == 64
-        assert 'last_modified' in repository
+        assert datetime.strptime(repository['last_modified'], ISO_FORMAT)  # must not raise
         assert list_repo['encryption']['mode'] == 'repokey'
         assert 'keyfile' not in list_repo['encryption']
+        archive0 = list_repo['archives'][0]
+        assert datetime.strptime(archive0['time'], ISO_FORMAT)  # must not raise
 
         list_archive = self.cmd('list', '--json-lines', self.repository_location + '::test')
         items = [json.loads(s) for s in list_archive.splitlines()]
@@ -1856,6 +1861,7 @@ class ArchiverTestCase(ArchiverTestCaseBase):
         file1 = items[1]
         assert file1['path'] == 'input/file1'
         assert file1['size'] == 81920
+        assert datetime.strptime(file1['isomtime'], ISO_FORMAT)  # must not raise
 
         list_archive = self.cmd('list', '--json-lines', '--format={sha256}', self.repository_location + '::test')
         items = [json.loads(s) for s in list_archive.splitlines()]