[susan53]

No - I'd say I'll have to go at half past, but only if twelve o'clock had just been mentioned - eg :

A : John should arrive at about twelve.
B : Great. I'll have to go at half past, but at least I'll get to see him.

[PYC]

So, you are saying that if I want to tell the time 12.30pm and the listener knows the hour, I can say that it's 30 till (instead of 30 till 1) but not half (half one or half past twelve) ?

[PYC]

Does that mean that even if the listener knows the hour, I can't say that " I'll have to go at half?" to mean that " I'll have to go at half one" as what "I'll have to go 30 till 1" can be replaced with " I'll have to go at 30 till" ?