diff --git a/src/borg/helpers.py b/src/borg/helpers.py
index d6720854c..fed3b769b 100644
--- a/src/borg/helpers.py
+++ b/src/borg/helpers.py
@@ -2212,7 +2212,7 @@ def basic_json_data(manifest, *, cache=None, extra=None):
             'mode': key.ARG_NAME,
         },
     })
-    data['repository']['last_modified'] = format_time(to_localtime(manifest.last_timestamp.replace(tzinfo=timezone.utc)))
+    data['repository']['last_modified'] = isoformat_time(to_localtime(manifest.last_timestamp.replace(tzinfo=timezone.utc)))
     if key.NAME.startswith('key file'):
         data['encryption']['keyfile'] = key.find_key()
     if cache: