I agree that honesty is usually the best policy. I think that you should tell them that you have an important personal appointment and that you need to leave early. Tell them that you are willing to make up the time you miss by staying late or arriving early if they want.
Here's a good excuse if you want dishonesty: It's tax time and you need to meet with an accountant. I don't condone this and if you get caught, I will disavow any knowledge of your activities and this proposed covert action. Good luck! This tape will self destruct in five seconds.
