answer:It’s definitely the professional, well-mannered thing to do, to let your prospective employer know of any pre-arranged responsibilities you have. No, you don’t have to tell them why. Just let them know, “I apologise for not mentioning it before, but if it isn’t too inconvenient I have an earlier commitment that will require my being out of town from date-X to date-Y.” They will let you know what arrangements will need to be made.