User Event Script to Set Due Date on Vendor Bill Reverts to the Current Date

Hi,

I have a User Event script which will get a specified date from the PO and set it as the Due Date on the Vendor Bill. The script successfully sets the Due Date field, however after the record fully loads the field changes back to todays date. Any thoughts on how I can make the updated date stick to the field? Script and video here.

function vendBillDueDate(type){

nlapiLogExecution('DEBUG','I am running!');

  var ponum = nlapiGetFieldValue('podocnum');

nlapiLogExecution('DEBUG','My PO #'+ponum);

  if(ponum != null && ponum != ''){

    var poRec = nlapiLoadRecord('purchaseorder', ponum);

var poTerms = poRec.getFieldText('terms');