Uses the jQueryUI datepicker and the JavaScript date object to calculate the number of business days remaining between today and whatever date is picked with the datepicker.

businessDaysRemaining.js

$(function() {
  $( "#datepicker" ).datepicker({
    onSelect: function(){
      // Gets the information from the picked date
      var picked = $("#datepicker").datepicker('getDate');
      var ddPicked = $("#datepicker").datepicker('getDate').getDate();
      var mmPicked = $("#datepicker").datepicker('getDate').getMonth();
      var yyyyPicked = $("#datepicker").datepicker('getDate').getFullYear();
      var dotwPicked = $("#datepicker").datepicker('getDate').getDay();

      // Set up today's date
      var today = new Date();
      var date = today;	
      var remaining = 0;			

      var dd = today.getDate();
      var mm = today.getMonth();
      var yyyy = today.getFullYear();
      var dotw = today.getDay();

      // Checks for whether the date has already occured
      if(picked < today){
        alert("Yo dawg, that date has already passed.");
        remaining = 0;
      }
      
      else{
        
        // Loops through dates
        while(date < picked){
          
          // Sets up the date that the loop is currently at
          var dd = date.getDate();
          var mm = date.getMonth();
          var yyyy = date.getFullYear();
          var dotw = date.getDay();
          
          // Adds a day to remaining days if the day isn't a Saturday or a Sunday
          if(dotw != 0 && dotw != 6){
            remaining++;
          }
          
          dd++;
          
          // Checks for whether the month is a February 
          if(mm == 1){
            
            // Checks for leap year
            if(yyyy % 4 == 0){
              
              // If so, there are 29 days in February and goes to the next month
              if(dd == 29){
                mm++;
                dd = 0;
              }
            }
          
            // If not, there are 28 days in February and goes to the next month
            else{
              if(dd == 28){
                mm++;
                dd = 0;
              }
            }
          }

          // If the month has 30 days in it, goes to the next month
          else if(mm == 3 || mm == 5 || mm == 8 || mm == 10){
            if(dd == 30){
              mm++;
              dd = 0;
            }
          }

          // If the month has 31 days in it, goes to the next month
          else{
            if(dd == 31){
              if(mm == 11){
                yyyy++;
                dd = 0;
                mm = 0;
              }
            else{
              mm++;
              dd = 0;
            }
           }
          }

          // Set the current date of the loop
          date.setFullYear(yyyy, mm, dd);

        }
      }	
    }
  });
});