diff --git a/types/axis.d.ts b/types/axis.d.ts
index f670f72b5..0c4782733 100644
--- a/types/axis.d.ts
+++ b/types/axis.d.ts
@@ -337,7 +337,7 @@ export interface YTickConfiguration {
 	 * Set formatter for y axis tick text.
 	 * This option accepts d3.format object as well as a function you define.
 	 */
-	format?(this: Chart, x: number | Date): string | number;
+	format?: ((this: Chart, x: number) => string | number) | ((this: Chart, x: Date) => string | number);
 
 	/**
 	 * Setting for culling ticks.