Slack slash command returning how long till Trump is out of office

trumpCountdownSlash.js

Date.daysBetween = function (date1, date2) {
  //Get 1 day in milliseconds
  var one_day = 1000 * 60 * 60 * 24;

  // Convert both dates to milliseconds
  var date1_ms = date1.getTime();
  var date2_ms = date2.getTime();

  // Calculate the difference in milliseconds
  var difference_ms = date2_ms - date1_ms;

  // Convert back to days and return
  return Math.round(difference_ms / one_day);
}

function processEvent(event, callback) {
  var today = new Date();
  var leaves = new Date(2021, 0, 20); //01/20/2021 12:00:00
  var days = Date.daysBetween(today, leaves);
  console.log('Days till ' +
    leaves.toLocaleDateString() + ': ' +
    days);

  var response = {
    response_type: "in_channel",
    text: `${days} days left until Trump leaves office.`
  }
  callback(null, response);
}


exports.handler = (event, context, callback) => {
  const done = (err, res) => callback(null, {
    statusCode: err ? '400' : '200',
    body: err ? (err.message || err) : JSON.stringify(res),
    headers: {
      'Content-Type': 'application/json',
    },
  });
  processEvent(event, done);

};