It is the first time +从句的结构中,从句用【现在完成时】.
It was the first time +从句的结构中,从句用过去完成时.没有别的时态.
例如：It is the first time that Mr Smoth has visited China.这是史密斯先生第一次到中国来游览.
It was the first time that Mr Smoth has visited China.译文同上.