Refactor rate limiter and datetime formatting

src/ddns_service/__init__.py

@@ -13,6 +13,7 @@ __all__ = [
     "cleanup",
     "cli",
     "config",
+    "datetime_str",
     "dns",
     "email",
     "logging",
@@ -20,5 +21,8 @@ __all__ = [
     "models",
     "ratelimit",
     "server",
-    "validation"
 ]
+
+
+def datetime_str(datetime):
+    return datetime.strftime("%Y-%m-%d %H:%M:%S")