[whpalmer4]

What do you mean by you can't set the dates to before today's date? You aren't given a field in which you can enter dates, or you are given such a field but the date immediately changes to today's date after you enter it, or you are given such a field and you enter the date but it has no effect on the scheduling, or what?

A good plan of attack for any "why is my project [not] doing this?" question is to use Help->Send Feedback in the app to send an anonymized copy of your plan to the Omni support group. All of the action names and notes text are replaced with Task 1, Task 2, and xxxx xxxx xxx so you don't have to worry about confidential data leaking.