Login and Logout time calculation in asp.net using C#

Question

Hi,
  
       I have 2 tables Logintime and logouttime... 

    Login Time is saved like 12-46-13 ...it means 12hr-46min-13sec..

    Logout Time is saved like 13-5-2 ...it means 13hr-5min-2sec..  

     LoginTime and  LogoutTime  saved successfully..


    I just want to find the time difference between Logintime and logout time.
    
   that time difference will save in another table ..

    and also i need this....

    Suppose if the user logged In and logged out  couple or three times per day means how i find the time difference ......
   
     I tried to find the difference....But i faced error like this...

    I have error while converting this time format to string