Difference between initial time and final time code not working correcly!

I am writing VBA code to calculate the difference between initial time and final time. It works until the time span moves to 13 hours. My application is to calculate the difference between two times. I will never have a value that goes over 24hrs. It works when I give it the following values: Initial 6hr, 30min, AM, Final 11hr, 30min, AM. - I get the value 5hr 0min -correct! The following values do not work: Initial 6hr, 30min, AM, Final 7hr, 30min, FM - I should get 13hr, 0min; however it gives me 1hr, 0min -incorrect!
Anybody have any idea why it is not calculating correctly? Thanks
redsab