excel help please can you spare me some time

I am making a timesheet to record time online in the A coloum i have the user namee in the B coloum I have the date in date format In the C coloum I got the time logged on in the D couloum time logged of In the E coloum i got a formouler that takes the time logged of from the time logged on wand format it into hour and min and seconds this is ok until the start time is larger then stop time as in 23.30 - 01:30 ect
the next thing i want to achive is on want to work out how to have to more coloum which tell me how how many min at off peck time and how long at full rate time in my sceanerio if the user was online between 6am and 6 pm there min will be charged at 4p a min if online after 6pm and before 6am then min online will be charged at 1p a min so i would like aformuler that will work out this promblem any help will be appriciated a lote thank you in advanced for helping me love tess