function dateTupleToYYYYDashMMDashDD(dateTuple) {
  const monthStr = toTwoDigitString(dateTuple[1]);
  const dayStr = toTwoDigitString(dateTuple[2]);
  return `${dateTuple[0]}-${monthStr}-${dayStr}`;
}
Date format such as 2019-01-20
(i.e.,  the ISO format and the conversion target).