add unit test for issue#504 (#506)

Author: erossignon

test/issue-504-test.js

@@ -0,0 +1,18 @@
+"use strict";
+
+const { FakeTimers, assert } = require("./helpers/setup-tests");
+
+describe("issue #504", function () {
+    it("should not mutate Date class", function () {
+        const priorDate = new Date();
+        assert.equals(priorDate instanceof Date, true);
+
+        const clock = FakeTimers.install();
+
+        const afterDate = new Date();
+        assert.equals(priorDate instanceof Date, true);
+        assert.equals(afterDate instanceof Date, true);
+
+        clock.uninstall();
+    });
+});