datepicker: altField + 3 days (date range)

datepicker: altField + 3 days (date range)


I'm currently working on a tool that allows the user to select a date range using two date fields:
- date from
- date until

I want the 'until' date field to be populated automatically after a 'from' date has been selected with the 'from'' date, + a few days (let's say 3). I don't think the default functionality supports this behaviour but I can't imagine no one has ever run into this problem before.

Does anyone know how I can realize this behaviour?