if(period == 'Every 30 Minutes') {
    cron = '0 ' + System.now().addMinutes(30).minute() + ' * * * ?';
}
else if(period == 'Hourly') {
    cron = '0 ' + System.now().minute() + ' * * * ?';
}
else if(period == 'Daily') {
    cron = '0 0 ' + System.now().addHours(1).hour() + ' *' + ' *' + ' ?';
}