Approval configuration- expression/function to build

Summary

Need help configuring the resignation approval so that employees cannot enter a termination date that is less than 14 days away from the submission date.

Content

Hi everyone, we have a requirement for our project which is the following: 

In this role, When I submit a resignation, I will be restricted from submitting my resignation IF the termination date is less than 14 days from the date of submission, so that employees cannot submit last minute self initiated terminations without contacting HR. 

In theory I believe the the expression of this rule would be something like:

IF termination_Date is BETWEEN current date and (current date+ 14)