Calendar::formatDate
A method that formats a date into a string. It inherits the locale and time zone settings of the calendar it’s called on.
calendar.formatDate( date, settings )
date can be a Date Object or something that will parse into a Date Object.
settings is an object that holds any of the date format config options.
Example:
import { Calendar } from '@fullcalendar/core';
import dayGridPlugin from '@fullcalendar/daygrid';
document.addEventListener('DOMContentLoaded', function() {
  var calendarEl = document.getElementById('calendar');
  var calendar = new Calendar(calendarEl, {
    plugins: [ dayGridPlugin ],
    timeZone: 'UTC',
    locale: 'es'
  });
  var str = calendar.formatDate('2018-09-01', {
    month: 'long',
    year: 'numeric',
    day: 'numeric'
  });
  console.log(str); // "1 de septiembre de 2018 0:00 UTC"
});