at half eight

Tom usually arrives at the office at half eight but because the traffic jam, he was over forty minutes late. -sentence with an error.

I think it should be: because of the traffic jam and shouldn’t it be: at half past eight?
or why is : at half eight correct?

Thanks!
;-]

I don’t use ‘half eight’, but it seems to me that I have heard it. It does not sound unnatural. Perhaps it is a BrE thing.

I don’t use it either. I have heard it too.