Flag where maternity start date is later than due date

We have identified a clerical error where an employee's maternity due date and leave start date were entered the wrong way round. Fortunately we picked up the issue before pay day, otherwise the employee would have had nil pay for the month.

By law maternity leave cannot start later than the employee's due date, so the system should have flagged that we'd entered due date as 27th July and leave start date as 1st October and not allowed the calculation.