Formulas counting time

I was on your Intermediate Excel 2007 course last week and wanted to learn how to create formulas for counting time. We have a spreadsheet monitoring our employees' working time but the formula doesn't seem to work correctly because even if the person works exactly 8.5 hrs 5 days a week the variation shows -0.5 which is of course wrong. How would you create such a formula? I would just like to have signing in / signing out times and then in the end of the week work out how much plus / minus the person has. Then every week I'd like to update a total variance sheet and monitor how much plus / minus each employee has. If you wish to have look at my spreadsheet (created by somebody else long time ago) I can email it to you.

Many thanks
Elina Puro