how to set format date in script google sheet

Question

i got a spreadsheet with automatical (trigger) sending mail each day by pdf with this code :

function sendmail1() {

  var email = "[email protected]"; 
  var feuille = SpreadsheetApp.getActiveSpreadsheet();
  var sujet = "Rapport journalier"; 
  var corpsDuMessage = "<p>Bonjour,</p>Ci-joint rapport journalier d'hier.<p>Bonne journée.</p>";
  var contenant = DriveApp.getFileById(feuille.getId()).getAs("application/pdf");
  
  contenant.setName(feuille.getName() + ".pdf");
  
  if (MailApp.getRemainingDailyQuota() > 0) 
    GmailApp.sendEmail(email, sujet, corpsDuMessage, {
      htmlBody: corpsDuMessage,
      attachments:[contenant]     
    });  
}

i would like to set "sujet" adding a date in the cell A1. how ? and how to set the format ? don't want : Tue Sep 01 2020 00:00:00 GMT+0200 (heure d’été d’Europe centrale) i would like to have format YYYY-MM-DD 2020-09-01

thank your for your help

code2

function sendmail1() {

  var email = "[email protected]"; 
  var feuille = SpreadsheetApp.getActiveSpreadsheet();
  var sujet = "Rapport journalier"; 
  var corpsDuMessage = "<p>Bonjour,</p>Ci-joint rapport journalier d'hier.<p>Bonne journée.</p>";
  var contenant = DriveApp.getFileById(feuille.getId()).getAs("application/pdf");
  
  contenant.setName(feuille.getName() + ".pdf");
  
  if (MailApp.getRemainingDailyQuota() > 0) 
    GmailApp.sendEmail(
  email,
  sujet +
    Utilities.formatDate(
      feuille
        .getSheetByName('Sheet1')
        .getRange('A1')
        .getValue(),
      feuille.getSpreadsheetTimeZone(),
      'yyyy-MM-dd'
    ),
  corpsDuMessage,
  {/*....*/}
);
}

Answer 1

Use Utilities.formatDate():

GmailApp.sendEmail(
  email,
  sujet +
    Utilities.formatDate(
      feuille
        .getSheetByName('Sheet1')
        .getRange('A1')
        .getValue(),
      feuille.getSpreadsheetTimeZone(),
      'yyyy-MM-dd'
    ),
  corpsDuMessage,
  {/*....*/}
);