vb code for time subtraction

[rustyshackleford]

I have used a macro to insert the current time using now() function in columns b(clock in time) and column c(clock out time). I had to shut off autocalculation or the now function would keep changing the time stamp. how would i write vba code to figure total hours worked for the day in column D and then cumulative hours worked for the week in column E. i can't just use a formula because the worksheet needs to be protected at all times. I hope this makes sense and any help would be most greatly appreciated