cron every 30 minutes from a specific time code example

Example 1: crontab every 30 minutes in specific minute

// every 30 minutes
*/30 * * * *

// it depends on the crontab version you are using
// example: every 30 minutes at number '5'

// mode 1
5,35 * * * *

// mode 2
5/30 * * * *

Example 2: 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 * * *