Synchronize time in javascript with a good precision (>0.5s) (NTP-like)

Question

I\'m looking for a way to synchronize time between clients with a good precision (let\'s say 0.5 seconds at least).

I exclude using jsontime or exploiting timestamp

Answer 1

If you just want to sync the timeclock on several computers, NTP themselves recommend setting up your own timeserver.

Server Endpoint

Set up an api endpoint on your server i.e.

http://localhost:3000/api/time

returns:

status: 200, 
body: { time: {{currentTime}} }

How this is done will depend on the backend language that you're using.

Client Code

Given such an endpoint, this JS snippet I threw together will

1. Poll your server endpoint 10 times.
2. Attempt to compensate for latency by removing half of the round-trip time.
3. Report the average offset between the server and the client.

var offsets = [];
var counter = 0;
var maxTimes = 10;
var beforeTime = null;

// get average 
var mean = function(array) {
  var sum = 0;
  
  array.forEach(function (value) {
    sum += value;
  });
  
  return sum/array.length;
}

var getTimeDiff = function() {
  beforeTime = Date.now();
  $.ajax('/api/time', {
      type: 'GET',
      success: function(response) {
          var now, timeDiff, serverTime, offset;
          counter++;
          
          // Get offset
          now = Date.now();
          timeDiff = (now-beforeTime)/2;
          serverTime = response.data.time-timeDiff;
          offset = now-serverTime;
          
          console.log(offset);
          
          // Push to array
          offsets.push(offset)
          if (counter < maxTimes) {
            // Repeat
            getTimeDiff();
          } else {
            var averageOffset = mean(offsets);
            console.log("average offset:" + averageOffset);
          }
      }
  });
}

// populate 'offsets' array and return average offsets
getTimeDiff();

You can use this computed offset (just add it to local time), to determine a common "universal" time from each client's context.