in time [adv.及时] 是具有副词性质的介词短语，用语修饰行为动词。

e.g. He arrived at the airport in time.他及时赶到了机场。

timely [adj.及时的, 适时的]是个形容词，用于修饰名词。

e.g. The timely rain after a long time of drought will certainly bring on the crops.久旱之后的这场及时雨肯定会有助于作物的生长。