Excel 2007 : Time Formatting and getting a % difference

The difference is a basic B less A (true Date & Time values are just Numbers (24 hours = 1)
Note: if using 1900 Date System (PC default) you can't display negative values in true Time Format (unclear if this is something you need to account for or not)

Regards %, of what ? You should find you can calculate the % of the difference without issue - can you elaborate ?

The difference in C is:

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The % is:

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However, my point re: negative times holds true - in this case where Actual > Planned you have an issue assuming you're running 1900 date system as Col C will not display.

Assume then the example times you provide are reversed such that Planned < Actual (eg -0:03:00)
If you wish to show the time as negative (without reverting to 1904 Date System - read up before doing so) then an alternative:

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note at this point the value in Col C is a text string and not a true Time Value, subsequently:

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