I assume that your data has headers, so the data should start from 2nd Row.

Copy and paste the below formula in M2 cell.
=IF($L2-$C2=TIME(12,0,0),"",($L2-$C2)-TIME(12,0,0))

Select the M Column and Do Right ClickFormat
CellsNumberCategoryCustomType copy and paste the below format or
type it.

[h]:mm:ss

And give Ok…

Remember to Click Yes, if this post helps!

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(Ms-Exl-Learner)
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"Steve M" wrote:

Hi
I would like to calcualte the time over a permitted amount and display it in
a cell.

Column C is the start time dd/mm/yy hh:mm and Column L is the finish time
dd/mm/yy hh:mm. The alloted time is 12 hours for the job and and need column
M to show the amount of time used over the 12 hours.

Many Thanks.