Nested IF Driving me crazy

Hi there,

I am working with an existing Spreadsheet at work that is basically our schedule. Up until now we have had to enter all the hourly totals by hand. I am trying to automate the task which I have done fairly well for daily totals. However I need to find the hours worked after 3:00Pm for the second shift workers. The data looks a little like this

a b c d e f g ... p

1 Sat Sun Mon ... Total 2nd

2 Employee A 15:00 - 23:00 15:00 - 23:00 12:00 - 20:00 ...
3 Employee B 16:00 - 23:00 Vacation 12:00 - 20:00 ...
4 Employee C 12:00 - 20:00 12:00 - 20:00 15:00 - 20:00 ...
5 Employee D "Blank" "Blank" 15:00 - 23:00

So what I need to do is Calculate all hours worked after 15:00 ignoring anything that includes text. I was trying to use IF and ISNUMBER but it keeps giving me errors... Also if the cell is blank if trys to use a zero value and gives me negative numbers... Can anyone help me?

anyone? bueller???