I am trying to get an error message to pop up if date entered is less than 3 weeks from the current date. Can anyone provide javascript code for this?

You need to use a custom validation script like:

if(event.value != "")

{

var oEnteredDate = util.scand("d-mmm-yyyy", event.value); // convert entered value to JS date object;

if(oEnteredDate == null)

{

  app.alert("Entered date is not a valid date.", 1, 0);

  event.rc = false;

} else {

  var nEnteredDays = Math.floor(oEnteredDate.getTime() / (1000 * 60 * 60 * 24)); // convert entered date to days since Epoch date;

  var oSysDate = new Date(); // get current system dateobject;

  oSysDate.setFullYear(oSysDate.getFullYear(), oSysDate.getMonth(), oSysDate.getDate(), 0, 0 ,0, 0); // set date to midnight;

  console.println(oSysDate);

  var nSysDays = Math.floor(oSysDate.getTime() / (1000 * 60 * 60 * 24)); // convert system date to days since Epoch date;

  if(nEnteredDays - nSysDays < 27)

  {

   // current is less than 27 days from current date;

   app.alert("Enterd days is less than 3 weeks from the current date.\nPlease reeneter.", 1, 0);

   event.rc = false;

  } // less than 27 days;

} // valid date inputted;

} // end entry not null string;

You will need to adjust format used in the script for the entered date to match the format for your date field's format.