Verify swedish personal id

gistfile1.js

var numonly = /\d{1,12}/;

// personalid.verify
// -------------------
// Takes a personal number and checks it.
// true - personal number is well formed and complete
// false - personal number is not well formed
// null - personal number is well formed but not complete
var verify = function(num) {

  // empty string is undecided
  if(num.length === 0) {
    return null;
  }

  // non numerics is invalid
  if(num.match(numonly)[0] !== num) {
    return false;
  }

  // shorter than 8 is undecided
  if(num.length < 8) {
    return null;
  }

  // 8 or higher - evaluate date
  var year = parseInt(num.substr(0, 4), 10);
  var month = parseInt(num.substr(4, 2), 10) -1;
  var day = parseInt(num.substr(6, 2), 10);
  var date = new Date(year, month, day);
  if(date.getFullYear() !== year || date.getMonth() !== month || date.getDate() !== day) {
    return false;
  }

  // < 12
  if(num.length < 12) {
    return null;
  }

  // checksum
  return (num
    .substring(2)
    .split('')
    .reduce(function(tot, cur, index) {
      return tot + (parseInt(cur, 10) * (2 - (index%2)));
    }, '')
    .split('')
    .reduce(function(tot, cur) {
      return tot + parseInt(cur, 10); 
    }, 0) % 10) === 0;
};