JavaScript Countdown not working (showing NaN) in Firefox

Question

This timer countdown works fine in Chrome, but doesn't work in Firefox...it just shows up as NaN. Anyone know why this is happening?

var deadline = 'May 3 2017 08:30:00 CST-0500';
function time_remaining(endtime){
 var t = Date.parse(endtime) - Date.parse(new Date());
 var seconds = Math.floor( (t/1000) % 60 );
 var minutes = Math.floor( (t/1000/60) % 60 );
 var hours = Math.floor( (t/(1000*60*60)) % 24 );
 var days = Math.floor( t/(1000*60*60*24) );
 return {'total':t, 'days':days, 'hours':hours, 'minutes':minutes, 'seconds':seconds};
}
function run_clock(id,endtime){
 var clock = document.getElementById(id);
 
 // get spans where our clock numbers are held
 var days_span = clock.querySelector('.days');
 var hours_span = clock.querySelector('.hours');
 var minutes_span = clock.querySelector('.minutes');
 var seconds_span = clock.querySelector('.seconds');

 function update_clock(){
  var t = time_remaining(endtime);
  
  // update the numbers in each part of the clock
  days_span.innerHTML = t.days;
  hours_span.innerHTML = ('0' + t.hours).slice(-2);
  minutes_span.innerHTML = ('0' + t.minutes).slice(-2);
  seconds_span.innerHTML = ('0' + t.seconds).slice(-2);
  
  if(t.total<=0){ clearInterval(timeinterval); }
 }
 update_clock();
 var timeinterval = setInterval(update_clock,1000);
}
run_clock('clockdiv',deadline);