time.js

// Determines the current time (with a +/- 1 second tolerance)
// by making HTTP requests to the hosts specified in hostnames.txt.
// 
// Assumes unanimity. If a single host's clock is off by more than
// two seconds, it'll invalidate the whole operation.
//
// $ node time.js

const fs   = require('fs');
const http = require('http');


function getDateFromHost(hostname) {
  return new Promise(function(resolve, reject) {
    http.get({ hostname }, function(res) {
      resolve(res.headers.date);
    }).on('error', reject);
  });
}

function guessTimeFromInterwebz(hostnames) {
  return Promise.all(hostnames.map(getDateFromHost))
    .then(function(dates) {
      const times = dates
                      .filter(function(d) { return !!d; })
                      .map(   function(d) { return (new Date(d)).getTime() });

      const min  = Math.min.apply(Math, times);
      const max  = Math.max.apply(Math, times);
      const diff = max - min;

      if (diff <= 2000)
        return new Date((min + max) / 2);
      else
        return null;
    })
}


const hostnames = fs.readFileSync('hostnames.txt')
                       .toString()
                       .split("\n")
                       .filter(function(line) {
                         return line.length > 0;
                       });

guessTimeFromInterwebz(hostnames)
  .then(function(time) {
    if (time)
      console.log(time.toISOString());
    else
      console.error('Could not accurately determine the time.');
  })
  .catch(console.error);