gistfile1.js

// calculated Coordinated Mars Time (MTC)

var d = new Date();
var millis = d.getTime();
var jd_ut = 2440587.5 + (millis / 8.64E7);
var jd_tt = jd_ut + (35 + 32.184) / 86400;
var j2000 = jd_tt - 2451545.0;
var msd = (((j2000 - 4.5) / 1.027491252) + 44796.0 - 0.00096);
var mtc = (24 * msd) % 24;

// then output MTC with h_to_hms(mtc) defined as:

function h_to_hms(h) {
  var x = h * 3600;
  var hh = Math.floor(x / 3600);
  if (hh < 10) hh = "0" + hh;
  var y = x % 3600;
  var mm = Math.floor(y / 60);
  if (mm < 10) mm = "0" + mm;
  var ss = Math.round(y % 60);
  if (ss < 10) ss = "0" + ss;
  return hh + ":" + mm + ":" + ss;
}