Time difference

Hi there,

Could you please help me on below.

I have 2 fields in my source table:
Login time(varchar)
Logout time(varchar)

I am trying to calculate the difference of both times.

eg:
Login time : 09:17:53
Logout time: 09:53:40

Difference is: 00:35:27

Can anyone help me..

Thanks
Mohan