Can I change my start date?

[shesaminiaturephilosopher]

I've just been offered a job that I really want, but I can't start on the day they want me to because I'm having an operation a few days beforehand!
I did tell the interviewer this, and he said it would be fine, but it seems to have been forgotten about. I'd only need to extend the start date by a week. Will this be OK? How do I ask for it?

Just remind them first thing tomorrow that you can't start until a week later

It'll be fine. Just tell them as soon as possible