Excel: calculating with times

I have twoo fields: IS and SHOULD.

  07:00 - 06:00 = 01:00
  07:00 - 08:00 = 01:00
    -> but it should be -01:00

how can I calculate times in the correct
way ???
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I ran into this before. Went to Microsoft and they had two solutions:
1)Change the date system to the 1904 date system.
2)Use the following to change the displayed result

Problem with 2 is that it is now text and you won't be able to put it into another formula.

For more info look here:

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Actually if I remember right, I got "###########", not 1:00 for negative time. What version of Excel are you using? Hope the solution works for your case too.
The solution I provided should work for the following versions:

Microsoft Excel for Windows, versions 2.10c, 2.10d, 3.0, 3.0a, 4.0, 4.0a, 5.0, 5.0c
Microsoft Excel for the Macintosh, versions 2.20, 2.2a, 3.0, 3.0a, 4.0, 5.0, 5.0a
Microsoft Excel for Windows 95, versions 7.0, 7.0a
Microsoft Excel 97 for Windows
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