diff --git a/types/plugin/timezone.d.ts b/types/plugin/timezone.d.ts
index 015e45ba..55855856 100644
--- a/types/plugin/timezone.d.ts
+++ b/types/plugin/timezone.d.ts
@@ -5,11 +5,11 @@ export = plugin
 
 declare module 'dayjs' {
   interface Dayjs {
-    tz(timezone: string): Dayjs
+    tz(timezone?: string): Dayjs
   }
 
   interface DayjsTimezone {
-    (date: ConfigType, timezone: string): Dayjs
+    (date: ConfigType, timezone?: string): Dayjs
     guess(): string
     setDefault(timezone?: string): void
   }