The solution we hit on was to leverage Doctrine's system of event listeners to help us do the work. Doctrine allows you to register listeners with the entity manager that are called whenever certain events occur. We created an event listener that is triggered on the onFlush event.

Code is included for the event listener they created - a simple "onFlush" event that grabs the current entities from the manager, sets the date/time property to allow it to be changed (via Reflection) and updating it with the new cleaned format.