In our onboarding and offboarding templates we prompt the manager to select the employees expected Start Date or Expected Departure Date. We try to encourage these forms be submitted 2 weeks prior to either event but this happens in rare occassions so using a SLA to manage due dates doesn't work well. 

Is it possible to create an action that will look at one of these fields and then update the "Due by Date" field. I tried to use Custom Actions -> Timers but the DueBy field isn't available under the 'Field Update' section. Ideally I would want to also update the 'Due By' fields within all the associated Tasks so we can leverage the notifications when a task runs past due. This is critical as we lose track of pending tasks for these types of requests and end up chasing techs to close them.