There is limited built-in support for formatting date strings with timezones in ECMA-262, there is either implementation dependent toString and toLocaleString methods or toISOString, which is always UTC. It would be good if toISOString allowed a parameter to specify UTC or local offset (where the default is UTC).
Writing your own function to generate an ISO 8601 compliant timestamp with local offset isn’t difficult:
function toISOLocal(d) {
var z = n => ('0' + n).slice(-2);
var zz = n => ('00' + n).slice(-3);
var off = d.getTimezoneOffset();
var sign = off > 0? '-' : '+';
off = Math.abs(off);
return d.getFullYear() + '-'
+ z(d.getMonth()+1) + '-' +
z(d.getDate()) + 'T' +
z(d.getHours()) + ':' +
z(d.getMinutes()) + ':' +
z(d.getSeconds()) + '.' +
zz(d.getMilliseconds()) +
sign + z(off/60|0) + ':' + z(off%60);
}
console.log(toISOLocal(new Date()));