code: func (c *MockClock) Set(t time.Time) {
  	c.mutex.Lock()
  	defer c.mutex.Unlock()
  
  	if t.After(c.now) {
  		c.advanceTo(t)
  	} else {
  		c.now = t // move back in time
  	}
  }
docs: // Set the mock clock to a specific time.  NOTE: scheduled wakeup calls are not
  // modified when resetting the clock to an earlier time; for example, suppose
  // the current time is X, and there is a pending wakeup call at time X+1.  If
  // we reset the clock to X-2, the wakeup time will still be at X+1.