The Datepicker is straight up JQueryUI.  So head over to the API Docs to see how it can be customized.  Looks to me like beforeShowDay option is what you need. 

To implement that customization on the Skuid date picker - take a look at this forum post where Zach describes overriding the standard Datepicker. 

I used Zach’s post in the link above with JQuery’s minDate function to not allow dates before another date from a field on my model. This allows for a simple, user friendly way to prevent users from selecting an end date before the start date.