def format_time_trigger_string(timer_instance):
    """
    :param timer_instance: either instance of RepeatTimer or EventClock
    :return: human-readable and editable string in one of two formats:
     - 'at Day_of_Week-HH:MM, ..., Day_of_Week-HH:MM'
     - 'every NNN'
    """
    if isinstance(timer_instance, RepeatTimer):
        return TRIGGER_PREAMBLE_EVERY + str(timer_instance.interval_new)
    elif isinstance(timer_instance, EventClock):
        timestamps = [repr(x) for x in timer_instance.timestamps]
        return TRIGGER_PREAMBLE_AT + ','.join(timestamps)
    else:
        raise ValueError('Unknown timer instance type {0}'.format(timer_instance.__class__.__name__))