Finding days between dates help

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I'm back again. I'll learn this eventually, I hope.

So I want to know the avg number of days from when a submission is completed, to the time the submission hits a specific CSW step called HL Review.

This is what I tried with no success:

Avg(DISTINCT TIMESTAMPDIFF(SQL_TSI_DAY, "Submission Dates"."Submission Completed Date", FILTER("Submission CSW Status - Historical"."Historical Status Event Date" USING ("Submission CSW Status - Historical"."Historical Step Name" = 'HL Review'))))

But it doesn't take that. I don't think it likes Filter in a math formula.