diff --git a/types/options.d.ts b/types/options.d.ts
index a3068b2ec..5d61f3c99 100644
--- a/types/options.d.ts
+++ b/types/options.d.ts
@@ -1193,7 +1193,7 @@ export interface SubchartOptions {
 				 * Use custom format for x axis ticks - see 'axis.x.tick.format' option for details.
 				 */
 				format?: string
-					| ((this: Chart, x: number | Date) => string | number)
+					| ((this: Chart, x: Date) => string | number)
 					| ((this: Chart, index: number, categoryName: string) => string);
 				/**
 				 * Show or hide x axis tick line.