How to silently saving Acrobat form file by adding datetime combination to existing filename.

Hello to everyone;
When after filling the dynamic acrobat form and silently save the which is entered data in a separate folder so far everything is fine.
This data file gets its name from the serial number field.
For example: PR-345675.fdf
But how to save it silently in a different folder with the current filename+date+time combination after that flatten the form completely.
Because I want to keep the main dynamic form layout file be safe.
I want a result like this.
For example: PR-345675_300821_022956.pdf
I do all this with the Acrobat JavaScript code which is assigned to

Code for Folder Level

MySaveAs.js (for silently save)

var mySaveAs = app.trustedFunction(

  function(oDoc, cPath, cFlName) {

  // Ensure path has trailing "/"

  cPath = cPath.replace(/([^/])$/, "$1/");

  try {

  app.beginPriv();

  oDoc.saveAs({cPath: cPath + cFlName});

  app.endPriv();

  } catch (e) {

  app.alert("Error During Save - " + e);

  }

  }

);

Config.js (For date time)

//DateTime function
function myDateString() {

return util.printd("ddmmyy_HHMMss", new Date());

}

Code for Clickable Button

1) Save fdf

var directory = "/Users/koyuncu/Documents/Charity-fdf/"

if (typeof(myExportFDF) == "function") {

  myExportFDF(this, directory, this.getField("SerialNumber").value + ".fdf");

} else {

  app.alert("Missing Save Function. Please contact forms administrator ");

}

2) Save pdf (Fully Flatten Form)

for (var i=0; i<this.numFields; i++) {
    var f = this.getField(this.getNthFieldName(i));
    if (f==null) continue;
    f.readonly = true;
}
this.removeField("Print");
this.removeField("Clear");
this.flattenPages()

var directory = "/Users/koyuncu/Documents/Charity/"

if (typeof(mySaveAs) == "function") {

  mySaveAs(this, directory, this.documentFileName + myDateString()+ ".pdf");

} else {

  app.alert("Missing Save Function. Please contact forms administrator ");

}

But I have small issue:
File name became like that: PR-345675.pdf300821_022956.pdf.
I don't want the extension ".pdf" repeated twice here in the final filename.

Thank you in advance for your help.