Add number of days from a date field into another field

Add number of days from a date field into another field

Hi,

I have a date field called Date Valid SSI Submission Received and I need to add another date field called 70 Day from Application.

I want the date field  called '70 Day from Application' to automatically work out and populate the date 70 days after the date that is inputted into D ate Valid SSI Submission Received .  

Please can you advise how I  can   do this?

Kind regards