semihourly

Adjective

 * 1)  Half-hourly: occurring every thirty minutes.
 * The warship rang its semihourly bell, announcing it was now half past 10 o'clock.
 * 1)  Roughly (but not exactly) hourly: occurring around every sixty minutes.

Adverb

 * 1)  Half-hourly: at intervals of 30 minutes.
 * Samples were collected semihourly (on the hour and half hour) throughout the study.
 * 1)  Roughly (but not exactly) hourly: at intervals of around 60 minutes.