How to calculate the difference between two times

I have a data in sql having varchar(50) as datatype now in crystal report i wnat to find the differences between two times like the employee checkin time vs checkout time how to do that i have tried cdate,tonumber,datevalue and many other functions but none of them work i do not want to change the datatype of my table if i change the data type and then use datediff it works .

 

Thanks