setMockDate.js

function setMockDate() {
  class MockedDate extends Date {
    constructor(...args) {
      const [firstArg, ...restArgs] = args;
      firstArg === undefined
        ? super("2020-09-01T12:00:00.000+09:00", ...restArgs)
        : super(...args);
    }
  }
  const {Date: ProperDate} = globalThis;
  globalThis.Date = MockedDate;
  return ProperDate;
}