Found this on some blog.  My notes aren’t complete.  But the results are listed.

Q: “I would like to view all entries within the last 30 days or greater.

A: Temporarily create a text column [Today]. You will need to delete this column to see if the formula works. Every time you edit a formula that references [Today] you will have to… “  <don’t know where rest of text is>

“You can now filter a list to show any items where the [calc column] equals one.”

This was used in an Calendar.

“=IF(([Today]-30)>([Start Time]-30),1,0)”

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