diff --git a/types/is-date-object/index.d.ts b/types/is-date-object/index.d.ts
index 08885dce51..4564b5e34b 100644
--- a/types/is-date-object/index.d.ts
+++ b/types/is-date-object/index.d.ts
@@ -2,7 +2,12 @@
 // Project: https://github.com/ljharb/is-date-object
 // Definitions by:  Adam Zerella <https://github.com/adamzerella>
 // Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
+// TypeScript Version: 3.3
 
-declare function isDateObject(value?: any): boolean;
+type InvalidTypes = undefined | null | boolean | string | number | [] | {};
+
+declare function isDateObject(value: Date): true;
+declare function isDateObject(value: InvalidTypes | unknown): false;
+declare function isDateObject(value?: unknown): false;
 
 export = isDateObject;
diff --git a/types/is-date-object/is-date-object-tests.ts b/types/is-date-object/is-date-object-tests.ts
index 68fb22d684..9946c42d54 100644
--- a/types/is-date-object/is-date-object-tests.ts
+++ b/types/is-date-object/is-date-object-tests.ts
@@ -7,6 +7,7 @@ isDate(false);
 isDate(true);
 isDate(42);
 isDate('foo');
+isDate((): void => {});
 isDate([]);
 isDate({});
 isDate(/a/g);