Please Help I Need Some Advice

OK. I am going to interview for a clinic job sometime this week or next week. I believe they have a Per diem position available. My question is how does per diem work in clinics? Do you make your own hours??
This is my dilemna, my parents retired about a year ago and live out of the country. I would like to visit them twice a year for about four weeks. So I would want to have two months of per year. Lets say January and July. This is supper important to me, I am very close with them and I want to be able to visit them as much as posible. I would be willing to give up benefits or whatever I have to do to be able to visit them twice a year. Now I dont know if I should bring this up in the interview or wait a couple of months and then ask. Please help! I dont know what to do. In situations like these is can you negotiate with your boss?? has anybody been in my situation?? what did you do??