fix: date prototype when using setSystemTime (#4584)

Author: spiroka

packages/vitest/src/integrations/mock/date.ts

@@ -55,12 +55,12 @@ class MockDate extends RealDate {
         break
     }
 
+    Object.setPrototypeOf(date, MockDate.prototype)
+
     return date
   }
 }
 
-// MockDate.prototype = RealDate.prototype
-
 MockDate.UTC = RealDate.UTC
 
 MockDate.now = function () {

test/core/test/date-mock.test.ts

@@ -33,4 +33,10 @@ describe('testing date mock functionality', () => {
 
     expect(Date.now()).toBe(timeStrMs)
   })
+
+  test('date prototype is correct', () => {
+    vi.setSystemTime(new Date(2000, 1, 1))
+
+    expect(new Date()).toBeInstanceOf(Date)
+  })
 })