Converitng Numbers into times for costings issue

Hi,
My name is Scott, and i am working on a scheduling system that costs itsef automatically. The aim is the when I draw up schedules for employees, i input the hour they start and the hour they finish. i.e.
A B
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1 * M o n d a y *
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2 *Start * Finish *
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3 Employee A * 0900 * 1300 *
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From the start and finish times I work out the number of hours worked per day (minus 1 hour for lunch if working over 8 hours). Using the following equation =IF(B3-A3>800,B3-A3-100,B3-A3). (I have using numbering instead of time as I am not sure how to convert time in numbers for costings). My problem is converting quarters in time to quarters in numbers for costings. ie schedules 0915 - 1330, needs to be 0925 - 1350, as the schedules would be misleading if written the send way.

So if you can help, or need more info, please leave a message and i can email the worksheets over.

Thank you

Scott Pattison