ntp client

index.js

const dgram  = require('dgram');
const socket = dgram.createSocket('udp4');

const port = 123;
const host = 'pool.ntp.org';
const data = Buffer.alloc( 48 );

// RFC 2030 header stuff...
// this all gets packed into a single byte that looks like this:
//
// 0 1 2 3 4 5 6 7
// +-+-+-+-+-+-+-+-+
// |LI | VN  |Mode |
// +-+-+-+-+-+-+-+-+

// Leap indicator – no warning = 0 (2 bits, shift 6)
const LI     = 0 << 6;
// Version Number – IPV4-only = 3 (3 bits, shift 3)
const VN     = 3 << 3;
// Mode – client = 3 (3 bits, shift 0)
const MODE   = 3 << 0;
// bitwise OR each value to get 0b00011011, or 27
const HEADER = LI | VN | MODE;

// all we need here is the first byte, everything else is zeroed out...
data.writeUInt8( HEADER, 0 );

socket.on( 'message', msg => {
  // integer ms since Jan 1 1900
  const ms       = msg.readUInt32BE( 40 ) * 1000;
  // fractional ms in addition to above
  const fraction = msg.readUInt32BE( 44 ) * 1000 / Math.pow( 2, 32 );
  const time     = ms + fraction;
  const date     = new Date('Jan 01 1900 GMT');

  date.setUTCMilliseconds( time );

  console.log( date );

  socket.close();
});

socket.send( data, port, host );