Date#toString with provided UTC offset.

blergh.js

var ofs = function(s) {
  var minute_ofs  = 1000*60*parseInt(s.slice(-2), 10)
    , hour_ofs    = 1000*60*60*parseInt(s.slice(-4,3), 10)
    , sign        = s.charAt(0) === '+' ? 1 : -1

  return sign * (minute_ofs + hour_ofs) 
}

Date.prototype.toString = function(ts, offset) {
  offset === undefined ?
    (offset = ts, ts = +this):
    null
  offset = offset || '+0000'
  ts = ts || +this

  var d = new Date(ts + ofs(offset))
  return d.toUTCString().slice(0, -4)+' '+offset
}

console.log(''+new Date())

console.log(new Date().toString('-0500'))
console.log(new Date().toString('-0600'))
console.log(new Date().toString('-0700'))