fix: add well known symbol method

src/CalendarDate.ts

@@ -34,6 +34,13 @@ export class CalendarDate {
    */
   readonly weekday!: number;
 
+  /**
+   * Customizes the default string description for instances of `CalendarDate`.
+   */
+  get [Symbol.toStringTag]() {
+    return 'CalendarDate';
+  }
+
   /**
    * Throws an Error for invalid inputs.
    *

test/CalendarDate.test.ts

@@ -1192,4 +1192,12 @@ describe('CalendarDate', () => {
       );
     });
   });
+
+  describe('Test of custom string description', () => {
+    test('the default string description of CalendarDate should be CalendarDate', () => {
+      const dateInstance = new CalendarDate('2021-01-01');
+      const tag = Object.prototype.toString.call(dateInstance);
+      expect(tag).toBe('[object CalendarDate]');
+    });
+  });
 });