time-calculator.js

function time_to_JD() {
  var timezone = document.getElementById("timezone").value;
  var year = document.getElementById("year").value;
  var month = document.getElementById("month").value;
  var day = document.getElementById("day").value;
  var hour = document.getElementById("hour").value;
  var minute = document.getElementById("minute").value;
  var second = document.getElementById("second").value;
  var time = new Date(
    Date.UTC(
      year,
      month - 1,
      day,
      Number(hour) - Number(timezone),
      minute,
      second,
      0
    )
  );
  var JD = document.getElementById("JD");
  JD.value = 2440587.5 + time.getTime() / 86400000;
}

function time_now() {
  var today = new Date();
  var h = today.getHours();
  var m = today.getMinutes();
  var s = today.getSeconds();
  m = check_time(m);
  s = check_time(s);
  document.getElementById("clock").innerHTML = h + ":" + m + ":" + s;
  t = setTimeout(function () {
    time_now();
  }, 100);
}
function check_time(i) {
  if (i < 10) {
    i = "0" + i;
  }
  return i;
}

function time_now_to_JD() {
  var time_now = new Date();
  var JD_now = document.getElementById("JD_now");
  JD_now.value = 2440587.5 + time_now.getTime() / 86400000;
}