Calculating time difference?

hi guys,
I have a excel sheet with staff details(employee id and name) and their entry and exit timings.But the entry and the exit timings are in the same cell.And also sometimes the entry time is in AM and the exit in PM,sometimes the entry is in PM and the exit is in AM(the next day).I need to calculate the time difference between these 2 times.
I made a sheet by splitting the time using right and left command in excel and converting that string into number and finding time difference.but this thing is not working for all the entry and exit times.
Can anyone help me with a good solution.
Please find the attached file for details.
Thanks.