This field is to display the "date" value when this Opportunity's Stage value contains "Closed ". and at the same time Opportunity's Stage value not contains Closed means this field needs to be empty/blank. Here Opportunity object Stage field datatype is picklist
IF((CONTAINS(TEXT(StageName),"closed")),NULL,Today())
Here am getting date value. but when Opportunity's Stage value not contains "Closed" means date field is not empty