<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def is_expired(self):
        '''
        Returns true if the time is beyond the window
        '''
        if self.window is not None:
            return (self._time() - self.start_time) >= self.window
        return False