standard template for such issue:

來源: 於 2011-06-19 20:56:20 [博客] [舊帖] [給我悄悄話] 本文已被閱讀：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.

這才是請人給麵子的句子。至於以後有沒有加班，老板是不會在乎的。