Advanced Date Picker for Delivery Date

I've set up an advanced date picker custom field. For the most part it's working fine, but my client would like to have it behave in a specific way.

We have two shipping methods: standard and expedited. We'd like the default date to be set 8 days in the future (similar to the way the waiting days option works), but still allow users to select dates sooner if using the expedited shipping option. With the waiting days enabled, the earlier dates are grayed out and can't be selected.

Can the default date field be used in such a way that it returns a date X days ahead, rather than a specific date?
Or, is there a way I could set up two versions of the date picker field and have them swapped based on the shipping method selected?

Thanks in advance!