System Prompts when start date changes to alert that leave is no longer accurate

What I want it to do - Prompt admin users when start dates are changed with a message to advise manual -re-calculation is now required.

Why - If it had a prompt, the errors would not occur as it would save a lot of time trying to retrospectively work everything out manually, and after being told by staff that their holidays are incorrect, which doesn't look good as we have 'trusted' the system . We use a system to remove manual admin.

Who would use - my HR and I am sure other customers would appreciate this too