Get the offset from GMT of the current date (equivalent to the format specifier 'O').
@param {Date} date The date
@param {Boolean} [colon=false] (optional) true to separate the hours and minutes with a colon.
@return {String} The 4-character offset string prefixed with + or - (e.g. '-0600').
function(date, colon) {
        var offset = date.getTimezoneOffset();
        return (offset > 0 ? "-" : "+")
            + Ext.String.leftPad(Math.floor(Math.abs(offset) / 60), 2, "0")
            + (colon ? ":" : "")
            + Ext.String.leftPad(Math.abs(offset % 60), 2, "0");
    }