Javascript: convert 24-hour time-of-day string to 12-hour time with AM/PM and no timezone

Nothing built in, my solution would be as follows :

function tConvert (time) {
  // Check correct time format and split into components
  time = time.toString ().match (/^([01]\d|2[0-3])(:)([0-5]\d)(:[0-5]\d)?$/) || [time];

  if (time.length > 1) { // If time format correct
    time = time.slice (1);  // Remove full string match value
    time[5] = +time[0] < 12 ? 'AM' : 'PM'; // Set AM/PM
    time[0] = +time[0] % 12 || 12; // Adjust hours
  }
  return time.join (''); // return adjusted time or original string
}

tConvert ('18:00:00');

This function uses a regular expression to validate the time string and to split it into its component parts. Note also that the seconds in the time may optionally be omitted. If a valid time was presented, it is adjusted by adding the AM/PM indication and adjusting the hours.

The return value is the adjusted time if a valid time was presented or the original string.

Working example

(function() {

  function tConvert(time) {
    // Check correct time format and split into components
    time = time.toString().match(/^([01]\d|2[0-3])(:)([0-5]\d)(:[0-5]\d)?$/) || [time];

    if (time.length > 1) { // If time format correct
      time = time.slice(1); // Remove full string match value
      time[5] = +time[0] < 12 ? 'AM' : 'PM'; // Set AM/PM
      time[0] = +time[0] % 12 || 12; // Adjust hours
    }
    return time.join(''); // return adjusted time or original string
  }

  var tel = document.getElementById('tests');

  tel.innerHTML = tel.innerHTML.split(/\r*\n|\n\r*|\r/).map(function(v) {
    return v ? v + ' => "' + tConvert(v.trim()) + '"' : v;
  }).join('\n');
})();

<h3>tConvert tests : </h3>
<pre id="tests">
  18:00:00
  18:00
  00:00
  11:59:01
  12:00:00
  13:01:57
  24:00
  sdfsdf
  12:61:54
</pre>

To get AM/PM, Check if the hour portion is less than 12, then it is AM, else PM.

To get the hour, do (hour % 12) || 12.

This should do it:

var timeString = "18:00:00";
var H = +timeString.substr(0, 2);
var h = H % 12 || 12;
var ampm = (H < 12 || H === 24) ? "AM" : "PM";
timeString = h + timeString.substr(2, 3) + ampm;

http://jsfiddle.net/Skwt7/4/

That assumes that AM times are formatted as, eg, 08:00:00. If they are formatted without the leading zero, you would have to test the position of the first colon:

var hourEnd = timeString.indexOf(":");
var H = +timeString.substr(0, hourEnd);
var h = H % 12 || 12;
var ampm = (H < 12 || H === 24) ? "AM" : "PM";
timeString = h + timeString.substr(hourEnd, 3) + ampm;

http://jsfiddle.net/Skwt7/3/

toLocaleTimeString() makes this very simple. There is no need to do this yourself anymore. You'll be happier and live longer if you don't attack dates with string methods.

const timeString = '18:00:00'
// Append any date. Use your birthday.
const timeString12hr = new Date('1970-01-01T' + timeString + 'Z')
  .toLocaleTimeString({},
    {timeZone:'UTC',hour12:true,hour:'numeric',minute:'numeric'}
  );
document.getElementById('myTime').innerText = timeString12hr

<h1 id='myTime'></h1>