[Brian]

Welcome to the forums, Jarret! If I'm understanding your question correctly, it sounds like the task in question requires 30 minutes of effort, and has an "end constraint" of the date for that deadline in question. You can set the end constraint in the Task Information inspector pane.

With those settings, the task will occupy 30 minutes of time, but can be scheduled at any point before the end constraint is reached. (Once any pre-requisite tasks are completed, of course.)

Does that help? If I misunderstood, let me know and I'll consult with someone that possesses more OmniPlan-fu than I. :-)