ServerTime

app.js

socket.on('ping', function(fn) {
    _.isFunction(fn) && fn(Date.now());
});

servertime.js

/**
 * @constructor
 * @param {number=} accuracy
 */
var ServerTime = function(accuracy) {
    this.accuracy = accuracy || 5;
    this.offset = null;
    this.latency = null;
    this.oncomplete = null;
};

// finds the averaged server's time
ServerTime.prototype.sync = function() {
    var self = this;
    this.internalSync(/*this.accuracy, */0, 0, 0, function(err, offset, latency) {
        self.offset = offset;
        self.latency = latency;
        _.isFunction(self.oncomplete) && self.oncomplete();
    });
};

// test case: server at +2000, client at -2000, rtt is 100
ServerTime.prototype.internalSync = function(/*runs, */total, totalOffset, count, callback) {
    //if (runs-- <= 0)
    if (count >= this.accuracy)
        return callback(null, totalOffset / count, total / count);
    var start = Date.now(), self = this; // start=-2000
    socket.emit('ping', function(time) { // +2050
        var end = Date.now(); // end=-1900
        var trip = (end - start) / 2; // trip=50
        var sTime = time + trip; // +2100
        var offset = sTime - end; // offset=4000
        self.internalSync(/*runs, */total + trip, totalOffset + offset, count + 1, callback);
    });
};

// takes the server's time, midrequest, and returns the equivalent time for the client
ServerTime.prototype.correct = function(server) { // 2150
    if (server.getTime)
        server = server.getTime();
    return server + this.latency - this.offset; // 2200
};

ServerTime.prototype.now = function() {
    return Date.now() + this.offset;
};

ServerTime.prototype.date = function() {
    return new Date(this.now());
};

var serverTime = new ServerTime();

socket.on('connect', _.bind(serverTime.sync, serverTime));