Javascript Months Array

'use strict';

(function(d){
var mL = ['January', 'February', 'March', 'April', 'May', 'June', 'July', 'August', 'September', 'October', 'November', 'December'];
var mS = ['Jan', 'Feb', 'Mar', 'Apr', 'May', 'June', 'July', 'Aug', 'Sept', 'Oct', 'Nov', 'Dec'];

d.prototype.getLongMonth = d.getLongMonth = function getLongMonth (inMonth) {
return gM.call(this, inMonth, mL);
}

d.prototype.getShortMonth = d.getShortMonth = function getShortMonth (inMonth) {
return gM.call(this, inMonth, mS);
}

function gM(inMonth, arr){
var m;

if(this instanceof d){
m = this.getMonth();
}
else if(typeof inMonth !== 'undefined') {
m = parseInt(inMonth,10) - 1; // Subtract 1 to start January at zero
}

return arr[m];
}
})(Date);