Time difference between two Date Time fields.

Hello,

Whenever i try want to calculate the time difference between a [Date Time] on/before 12th of every month to [Date Time] on/after 13th of every month gives error value. The actual issue is of the Date format.
For Example :

Cell A1 = 07-12-2014 12:12:08 -------> 12th July 2014
Cell B1 = 07/13/2014 13:04:13 --------> 13th July 2014
Cell C1 = B1 - A1 ---------------------> gives "#Value!"

Require output in HH:MM:SS format.
I have to replace manually the date format from either "-" to "/" or "/" to "-"

Kindly help..

Thanks Admin, I suppose by mistake i posted my query on the wrong Thread... Sorry for the Inconvenience...

Hope to get the solution to my query..