年代的缩写为什么是s

　　例句：

　　我和他初次见面早在1980年代。

　　I first met him way back in the 80s.

　　他开始谈论他叔叔那个年代的乌克兰。

　　He began to talk about the Ukraine of his uncle's day.

　　他们在战争年代开始赚大钱。

　　They began to make big money during the war.