diff --git a/types/timezone-js/index.d.ts b/types/timezone-js/index.d.ts
index 5a648ab41e..ee39a7ee7b 100644
--- a/types/timezone-js/index.d.ts
+++ b/types/timezone-js/index.d.ts
@@ -19,7 +19,7 @@ export declare class Date {
     setTimezone: (timezone: string) => void;
 
     // regular Date members
-    toString(): string;
+    toString(format?:string): string;
     toDateString(): string;
     toTimeString(): string;
     toLocaleString(): string;