ASP.NET

// verify string
if (/^\d\d-\d\d-\d\d$/.test(input)) {
   // get current time
   var today = new Date();
   // get current + 5 days
   var limit = new Date(today.getYear(), today.getMonth(), today.getDay()+5);
   // split input
   var arr = input.split('-');
   // create Date from input
   var inputDate = new Date(parseInt(arr[2],10), parseInt(arr[1],10)-1, parseInt(arr[0],10));
   // compare dates
   if (inputDate >= limit) {
      alert('Date is ok.');
   }
}