[Serusers] simple rating process

Hi I've looked at the serweb application for calculating time for each call, assuming INVITE/BYE matching, has anyone based on the details in the acc table worked on a simple rating script, which I could beg/steal/borrow :-) The problem I have is on-peak/off-peak , since I know when the call starts hence know which of the blocks (i.e on/off peak) it falls into, but I am getting confused as to howto calculate it. EG peak 1300 to 1500 , and call starts 14:45:00 and last for 16 mins hence finishes 16:01 hence 15 mins is peak and 1 minute is off peak, I just cant work out how to write the algo for this, so if someone has some pseudo code (or even real) that would be great. I have rating tables based on prefix all created, with the on-peak/off -peak entries, and the call time , as I have mentioned, can be pulled using the serweb logic... tks Iqbal