Hello, I need help writing a formula to calculate the amount of years a contact has held a position once the Position Start Date has been populated and then for the calculation to stop once the Position Stop Date field is populated. 

This is what I have so far, but i'm having trouble making the calculation stop once the position stop date field is populated

IF(ISBLANK( Position_Start_Date__c ), NULL, TEXT(FLOOR((Today()- Position_Start_Date__c)/365))& " "&"Years")

Hello, I need help writing a formula to calculate the amount of years a contact has held a position once the Position Start Date has been populated and then for the calculation to stop once the Position Stop Date field is populated. 

This is what I have so far, but i'm having trouble making the calculation stop once the position stop date field is populated

IF(ISBLANK( Position_Start_Date__c ), NULL, TEXT(FLOOR((Today()- Position_Start_Date__c)/365))& " "&"Years")