ISO8601DateWithTimeZone.js

/* keep a reference to original toISOString to use it into new method */
const oldToISOString = Date.prototype.toISOString
/* Define new method */
const toISOString = function toISOString(date) {
    if(!date) return date;
	/* extract GTM timezone from date.toString() */
    const regex = /(?:GMT)([-+]\d*)/gm;
    let gtm = regex.exec(date.toString())[1];
    gtm = [...gtm];
	/* insert ':' in time -0500 -> -05:00*/
    gtm.splice(3, 0, ':');
    gtm = gtm.join('')
	/* replace Z (UTC) to respective TimeZone */
    return oldToISOString.apply(date).replace('Z', gtm)
}
/* override toUSOString method using new toISOString */
Date.prototype.toISOString = function(){
    return toISOString(this);
}