def _get_trading_day_start_time(trading_day):
    """给定交易日, 获得交易日起始时间"""
    begin_mark = 631123200000000000
    start_time = trading_day - 21600000000000
    week_day = (start_time - begin_mark) // 86400000000000 % 7
    if week_day >= 5:
        start_time -= 86400000000000 * (week_day - 4)
    return start_time
