Auto create date

Auto create date

Hi , I have a question:
my events have a start date and end date. (normally 4-5 days apart)
I would like to create an automated field in events for departure and reentry... 
This is normally: departure 2 days before the start date and 1 day after the end date... is their a way i can have this done automaticlly?

The ideal scene would be that I have a date field for departure and it is automaticlly filled in with 2 days before but I would also be able to change this manually if needed
same thing for reentry/end date.

Thank you all in advance for your advice.
Benny