下面这句话中为什么用“at”而不用“in”？表示某一天不是用“in”或“on”么？

Question

He loved to be told that one of his imaginary products was temporarily out of stock and he would faithfully promise to call again at some future date （他爱听售货员说他想像出来的那种东西暂时脱销，于是他就煞有介事地许诺 改天再来光顾）——《新概念英语》
另外，表示不具体的某一天用in还是on？类似someday这种不具体的某一天

Answer 1

这个问题我很想回答，可我没学英语，为了奖品，我没办法，只能乱写🙂

Answer 2

ginary products was temporarily out of stock and he would faithfully promise to call a