crontab every 2 hours between code example

Example 1: crontab every 30 minutes between hours

// from 08h to 17h (until 17:30)
*/30 8-17 * * *

// from 08h to 17h, at number '5' (until 17:35)
5,30 8-17 * * *

Example 2: cron every two hours

0 */2 * * *