How to create a cronjob that will execute some code from 8:30AM to 8:14PM.
As a coder I would suggest the following method:
1 line inside your cron to start the script
Killing a script could have side effects: just maybe the script is currently doing something important. Killing it will without any mercy end it. Gracefully exiting a program is always better.
This link has some good info. Crontabs support ranges in a very straight forward way.
This will run once a minuet from 8:14 to 8:30
Check the Man page with man crontab next time it has some very good examples, including this one.
Cron is only used to start programs though, so make sure you don't overlap running instances of your program.
Ubuntu documentation CronHowto
Webmin has a gui for cron jobs (System --> Scheduled Cron Jobs)