Half time

There's a little cultural difference that I've never blogged about, but always throws me a tiny bit. Well, it threw me a lot when I first moved here.  It has to do with time. How hard can that be, right? I mean time is time! Well, yes. And no.

No. Not that Time.

I know it's because I have a tendency to over-think/complicate things sometimes. I'll give you a sample conversation.

Me: Remind me. What time is the plumber coming tomorrow?

Ross: Half Ten.

Me: (thinking, not aloud) Wait. Does that mean 9:30 or 10:30? I think it's 9:30. Right? Because it's half way to 10. Right? Wait. Half ten would be 5:00 so that can't be right. (out loud) ...10:30?

Ross nods.

This repeatedly confused me for WAY longer than I should admit. I know now what it means. And only second guess myself very infrequently. But it's definitely an interesting difference. I've never heard a British person say "thirty" as part of the time. It's always "half". I wonder why that is...

Any guesses?