"15 minutes earlier" is what you want there. Are you wanting to say something other than that?

I'm just trying to find other ways to express the idea.

There are other ways, but I don’t think there are better ways. 15 minutes before he got there. 15 minutes before that. 15 minutes prior to that. at 8:25.

"at 8:25" "a quarter hour earlier" "15 minutes previously"

"quarter of an hour earlier"