How do I set a UI Only Date Field's default value to be a Date in the past based upon the current Da

I have a field editor that has 2 UI Only fields Called Start Date and End Date.  I am able to set the End Date to Today by simply putting in the word TODAY in the fields Default Value.  However, I cannot figure out how to set the Start Date to be a date in the past based upon today’s date.  For Example I want the Start Date to be either the first date of the current month or 30 days from today’s date.