function calculateDate(date1, date2){
//our custom function with two parameters, each for a selected date
 
  diffc = date1.getTime() - date2.getTime();
  //getTime() function used to convert a date into milliseconds. This is needed in order to perform calculations.
 
  days = Math.round(Math.abs(diffc/(1000*60*60*24)));
  //this is the actual equation that calculates the number of days.
 
return days;
}
 
function rsfp_onSelectDate(date, type, args, calendar){
//RSForm!Pro specific calendar function.
//args = object, containing the date in the exact format needed for the above getTime() function.
//calendar = object, containing various properties of the calendar.
 
argsstr = String(args);
spldate = argsstr.split(",");
date1 = new Date(spldate[0],spldate[1]-1,spldate[2]);
//the selected date converted into proper format for calculations
 
date2 = '';
 
if(calendar.myid == 'cal5_0' && RSFormProCalendars[5]['datum1'].getSelectedDates() != '')
  date2 = new Date(RSFormProCalendars[5]['datum1'].getSelectedDates());
 
if(calendar.myid == 'cal5_1' && RSFormProCalendars[5]['datum0'].getSelectedDates() != '')
  date2 = new Date(RSFormProCalendars[5]['datum0'].getSelectedDates());
 
//the above if clauses, check from which of the two calendars the selection was made and assigns the ending date
//only when both dates are selected.
 
 
if(date2 != ''){
//the calculation will be done when the dates are selected.
  days = calculateDate(date1, date2);
 
  document.getElementById('anztage').value = days;
  //this will assign the days difference value to our textbox.
}
 
return true;
}