Разница дат в секундах

const getSecondsDiffBetweenDates = (dateInitial, dateFinal) =>
  (dateFinal - dateInitial) / 1000;

getSecondsDiffBetweenDates(
  new Date('2020-12-24 00:00:15'),
  new Date('2020-12-24 00:00:17')
); // 2