Date not older than another date field

Thank you Chris! I did a try with your formula but I do wrong somewhere 

ifElse(or(greaterThan(daysUntil($plannedstartdate_c),daysUntil($date_closed)),equal(daysUntil($plannedstartdate_c),daysUntil($date_closed))),"plannedstartdate_c","")

I have tried in the last section both with plannedstartdate and plannedstartdate_c

Can you see where it goes wrong?

The formula seems to be ok, which kind of field did you create and subject of such formula?

The formula seems to be ok, which kind of field did you create and subject of such formula?

Both fields are date fields- nots datetime.

It is in the module Opportunity

Best regards