Hi @Sue Hannan ,
JPD date fields are not currently supported within custom formulas at this time. We do already have an open feature request for this functionality entered with engineering. I'll add this question as a +1 to it.
In the meantime, an automation could be used as a workaround. I provided an example in this other Community Question: Formula field including project target / due date
If you can provide more about the use case, I may be able to test / share an automation that works for you!
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