currentDateTimeString diffTime diffTimeF getMiliseconds getSeconds isNull null operator!= operator< operator<= operator= operator== operator> operator>= setCurrent setTime_t TimeInfo TimeInfo ~TimeInfo

isNull() bool isNull ( ) const 检查时间对象是否为空 返回时间对象是否为空的标志。 返回 如果时间对象为空，返回 true；否则返回 false