server.js

// server.js
// where your node app starts

// init project
var express = require('express');
var app = express();

// enable CORS (https://en.wikipedia.org/wiki/Cross-origin_resource_sharing)
// so that your API is remotely testable by FCC 
var cors = require('cors');
app.use(cors({optionSuccessStatus: 200}));  // some legacy browsers choke on 204

// http://expressjs.com/en/starter/static-files.html
app.use(express.static('public'));

// http://expressjs.com/en/starter/basic-routing.html
app.get("/", function (req, res) {
  res.sendFile(__dirname + '/views/index.html');
});


// your first API endpoint... 
app.get("/api/hello", function (req, res) {
  res.json({greeting: 'hello API'});
});

// My code
app.get("/api/timestamp/:date_string?",function(req, res){
  var date; 
  const numRex = /^\d+$/;
  if (req.params.date_string == undefined)
    date = new Date();
  else if (numRex.test(req.params.date_string)){
    date = new Date(parseInt(req.params.date_string));
  }
  else {
      date = new Date(req.params.date_string);
  }
  res.json({"unix": date.getTime(), "utc" : date.toUTCString() });
});

app.use(function (req, res, next) {
  res.status(404);
  res.send('Not found');
});

app.use(function (err, req, res, next) {
  console.error(err.stack);
  res.status(500);
  res.send('Server Error');
});

// listen for requests :)
var listener = app.listen(process.env.PORT, function () {
  console.log('Your app is listening on port ' + listener.address().port);
});