You can display negative times if you use "1904 date system". You need to set that in a workbook before you put the dates in....otherwise the dates will change.....so in a blank workbook click on "Office" button at top left then

Excel Options > Advanced > scroll down to "When calculating this workbook" > tick "1904 date system"

Now copy in your dates....and you can use a simple subtraction formula, e.g. in C1

=B1-A1

custom format C1 as

[h]:mm;[Red]([h]:mm);h:mm

to show as required