Forcing system time for jest tests

jestSetSystemTime.js

/**
 * Set the system time for our tests.
 * This ensures that our tests can't fail based on the local time of the CI machine.
 * This guards against app code where the behavior changes based on the device time.
 * For example, a test might fail if we attempt to set the search time "before now".
 */
beforeAll(() => {
  // @TODO: 'modern' is default once we upgrade to Jest 27 https://jestjs.io/blog/2020/05/05/jest-26#new-fake-timers
  jest.useFakeTimers("modern");
  const JS_DATE_11_AM = new Date(new Date().setHours(11, 0, 0, 0));
  // This is a typical time during the workday, during our mock building working hours.
  const DEFAULT_TEST_TIME = JS_DATE_11_AM;
  jest.setSystemTime(DEFAULT_TEST_TIME);
});

afterAll(() => {
  // Undo the forced time we applied earlier, reset to system default.
  jest.setSystemTime(jest.getRealSystemTime());
  jest.useRealTimers();
});

// Why didn't this work?
// The new "modern" timer mocks in jest were breaking some internal async part of Redux Saga - it would never resolve.
// Switching to the default timer mock (`.useFakeTimers()`) fixes the issue, but that doesn't
// support setting the system time.