<select></select>
<output></output>

const options = {
  'осень': new Date(2022, 8, 1),
  'новый год': new Date(2023, 0, 1),
  'налоги': new Date(2023, 3, 1),
};

const select = document.querySelector('select');
const output = document.querySelector('output');

Object.keys(options).forEach(title => {
  const el = document.createElement('option');
  el.innerText = title;
  el.value = title;
  select.appendChild(el);
});

let current = select.value;

select.addEventListener('change', () => {
  current = select.value;
  update();
});

const update = () => {
  let s = Math.floor((options[current] - new Date())/1e3);
  output.innerText = s;
};

setInterval(update, 250);