on the half hour

Prepositional phrase

 * 1) Happening hourly, but at 30 minutes past each hour; ie, 08:30, 09:30 etc.
 * Trains leave here for New York every hour on the half hour.
 * 1) Happening on the hour and at 30 minutes after the hour.
 * 2)  At thirty minutes into a half.
 * 1)  At thirty minutes into a half.
 * 1)  At thirty minutes into a half.