When I go to configure a Schedule in the Azure management console, I\'m only given the option of scheduling with an absolute end date/time (or never ending) and an interval.
You can use the built-in scheduling which is more flexible than the Azure one. You can learn more about how that works from this blog post http://blog.amitapple.com/post/2015/06/scheduling-azure-webjobs/
The summary: create a file called settings.job that contains the following piece of json
{"schedule": "cron expression for the schedule"}
in your case the cron expression for "every 30 minutes from 8am to 6pm" would be 0,30 8-18 * * *
so the JSON you want is
{"schedule": "0,30 8-18 * * *"}
Keep in mind that this uses the timezone of the machine, which is UTC by default.