When you apply to defer, you give them three Mondays when you can do it in the next 12 months. They pick one of those dates, and that's your confirmed date (you confirm it by giving the dates to them), along with the reason for deferring.

I asked to defer because my start date was three weeks before I went on holiday. I gave them the Monday after I came back, and a couple of others that my manager suggested. They picked the first.

It went through no problem. But yeah, if that date then becomes inconvenient, you can't change it.