UserSchema.methods.formatDob = function (day, month, year) {
  let dob = new Date( parseInt(year), parseInt(month) - 1, parseInt(day), 0, 0, 0, 0 );
  return new Date(dob, '<YYYY-MM-DD>');
}

exports.register = async function(req, res, next) {
  try {
    // instance methods not available yet
    let user = db.User.create(req.body);
    // method now available but too late to modify before inserting

const { dobYear, dobMonth, dobDay } = req.body;
const dob = formatDob(dobDay, dobMonth, dobYear);
user = new db.User(); // can't pass in req.body here
// Now I have to manually set all the fields!
...
user.dob = dob;
user.save();