title: How to get the number of days in a month
body: <p>I make a countdown timer, I need to take the number of days in a month and subtract the days that have passed (for example: 30 days of September minus 8 days - the date is today).
  Now I have to enter it manually:</p>
  <pre><code>const end = new Date(2021, 8, 30, 13, 0,12, 12);
  </code></pre>