You’re getting an error because like I said, the FILTER part of the expression is built into the Slice functionality. The expression for a slice must return a TRUE or FALSE result. The FILTER expression returns a LIST from a table based off the provided condition.
Therefore, you cannot use FILTER in a Slice in this manner. And your Slice expression should just be [visiting date] >= (TODAY()-31), because this is what will produce a TRUE or FALSE result for if the row is to be included in the Slice.
LinkedIn
Twitter
