Wiktionary
on the half hour
adv. happening hourly, but at 30 minutes past each hour; ie, 08:30, 09:30 etc.
Usage examples of "on the half hour".
After that the tube runs one car on the half hour instead of every ten minutes.
By early afternoon, having rotated tasks on the half hour, we were beginning to wonder if some old pirate hadn't deep-sixed a busted cannon.
He'd counted on the half hour it took to bring the huge machine on-line.
KFI called it a naval assault, been playin' 'Anchors Aweigh' every hour on the half hour.