I want to make a field mandatory only on records created after a specified date. So if the opportunity created date is prior to current date then field wouldn't be mandatory but if opp. created date is = current date or later then field is mandatory. Is there a way to write that?
You can try validation rules like;
Error Condition :
AND (NOT( ISNULL(CreatedDate ) ), (TODAY() > DATEVEALUE(CreatedDate) ), ISNULL(f2__c ) )
NOT( ISNULL(CreatedDate ) ) : Will allow new records to be created without checking for f2__c
TODAY() > DATEVEALUE(CreatedDate) : This will check records that are not created today.