Errors with date validation script

Hello,

I'm attempting to use the script below to validate a date in a custom field using the OnFieldChanged function.  Once I have selected a date from the calendar to populate my field I want to check to see if the date is at least 2 weeks out from today's date.  Here is the section of the script I'm working on:

else if(name == 'custbody_propduedate')
{
  var PropDate = new Date();
  PropDate.getDate();
  PropDate = nlapiDateToString(PropDate);
  alert(PropDate);

  var DueDate = nlapiGetFieldValue('custbody_propduedate');
  //DueDate = nlapiDateToString(DueDate);
  alert(DueDate);

  var DateSub = 0;
  var DateSub = parseInt(DueDate - PropDate);
  alert(DateSub);

  //DateSub = DateSub/(1000*60*60*24);
  //alert(DateSub);