Calculate duration startDate & endDate from an HTML form

Im wondering if anyone can point me to some documentation on how to solve my issue.
Still new at native JS, and looking to do some date calculations.

Basically I need to calculate the duration between X and Y dates from and HTML forms input type date.

I have a rough sketch of my HTML and JS..
but still I cannot solve the issue without propper guidance and documentation.

       <div>
          <form>
              <label for="start-date"></label>
              <input type="date" id="start-date">

              <label for="end-date"></label>
              <input type="date" id="end-date">

              <label for="duration"></label>
              <input type="text" id="duration">
         </form>
       </div>

    const startDate = document.getElementById("start-date").value
    const endDate = document.getElementById("end-date").value
    let duration_from_form = document.getElementById("duration").value

    let duration = endDate - startDate;


    if (duration_from_form === ""){
        duration_from_form = duration;
    }
}

I know both of them a rough mockups and not ideal, but I’d love some help finding propper documentation on this matter.
Kind regards