How to Calculate Negative Times in Excel

1. Open your Excel 2010 worksheet. Click on an empty cell anywhere on the worksheet.
2. Type '-'1:30'' into the cell. The number '-.0625' should appear. Right-click the cell and choose 'Format Cells.' Choose 'Time' from the list on the left and click 'OK.' The number changes to '#####.'
3. Click on the 'File' tab at the top of the screen. Choose 'Options' from the list that appears.
4. Click 'Advanced' on the list on the left side of the Options window. Scroll down to the bottom of the 'Advanced' screen and look for the 'When calculating this workbook' heading. Click the box next to 'Use 1904 date system.'
5. Click 'OK' to close the Options window. The time that you entered now reads '-1:30.' You can perform normal mathematical operations on the number and it will behave exactly like a negative number would.