standard template for such issue:

来源: 于 2011-06-19 20:56:20 [博客] [旧帖] [给我悄悄话] 本文已被阅读：次

I have to leave 2 hours earlier due to ...., but I will make up (or "compensate", if you want to formalize) the hours some time next week.

这才是请人给面子的句子。至于以后有没有加班，老板是不会在乎的。