@clancularius Sorry for the trouble! When scheduling a project in OmniPlan, you can only edit the start or the end date for the project, not both (as OmniPlan is a tool for determining your project’s schedule, the other date value is calculated by OmniPlan). If you’re scheduling your project backwards for a fixed a date, you may want to switch to this scheduling mode in the OmniPlan Project Info Inspector. Doing so will allow you to input an end date, and OmniPlan will calculate your project’s start date instead.
Additional information about Fixed Start vs. Fixed End projects is available in this section of our manual:
Hope this helps!