Formula to return date value of the last record in a field (or to return the date value of the most recent date among records with unique dates in a Date fields)

Hello,

I have a Contacts table with an Interactions subtable. I wish to track unique interactions with different business contacts and remind myself when I should follow up with people. I'd like to display the date of the most recent interaction I've had with someone as a formula calculation.

I've tried last(Interactions.Date) and max(Interactions.Date), but they both show up empty. The Interactions table is not empty, and I am able to view an interation with the current contact in another field on the same form tab, so that's not the problem.

I'm unsure of how to reference the last record in the Interaction table associated with the current contact (or even better, the Interaction record with the most recent date, associated with the current contact) and return the date value of that record.

Thanks for any help you can give!