If the test fails, the result is TRUE.
If the test is met, then the result is FALSE.
The TestToPerform can be reference to cells or another calculation.
No special formatting is needed.
The date the book was Taken out is entered.
The period of the Loan is entered.
The date the book was returned is entered.
The =NOT() function has been used to calculate whether the book was returned within
the correct time, by adding the Loan value to the Taken date.
If the book was not returned on time the result Overdue is shown, otherwise OK is shown.