Oddly, we don't say "half after" but we do say "quarter after" and "quater of" or "quater to." (The latter interchangeable spoken, but "quarter to" in writing."
"Half past" isn't used very often.
I know it's very idiomatic, but you'd be more likely to hear "I'll be there about quarter after, maybe 6:30 if the traffic is bad." Or even "I'll be there about 6:30, maybe quarter o