Similarly, when I moved from the UK to Canada, people often didn't understand what I meant when I said it was "half ten", which is the common way of saying ten thirty, at least where I grew up.
How does one get to "half ten" in German? Is it simply starting from "half to ten"?
Never thought about it much but I think you're spot on. English uses "half past" and therefore "half 10" means 10:30, whereas most other languages use "half to" which causes "half 10" to mean 9:30.
One would think this should cause confusion for international meetings often enough to be common knowledge, but I didn't know until today...
English thinkers might also want to consider the fact that 0930 is half-way through hour 10
The only other English common case I can think of is the American “I could [not] care less” dropping “not” which is also confusing.