getSecondsFromToday.js

function getSecondsToday() {
  let now = new Date();

  let today = new Date(now.getFullYear(), now.getMonth(), now.getDate());

  let diff = now - today; // разница в миллисекундах
  return Math.round(diff / 1000); // получаем секунды
}

alert( getSecondsToday() );