You say that you like the job, but then again, you are applying for a different one. If you really "liked" the job, you would be looking forward to extending it to 8 months, not interviewing for a different one.
You need to do two things here: one is to be honest with yourself about what you are doing, and the second is to lie to your boss. A hospital appointment, a family problem, something unavoidable. Keep it simple, and don't tell more than a minimum: if they ask afterwards, say you would rather not talk about it. Don't look cheerful, as if the problem has gone away: you may need more days off for further interviews.