How do I get local time in ISO format in Javascript?

Question

I want to convert time in milisecond to my local time in ISO format.

let time = 1668268800000
console.log(new Date(time).toISOString())

However this do not output my ISO date in my local time.

Answer 1

ISO is UTC by definition. Consider using JS's internationalization library to get dates in local format.

You can also do the conversion yourself:

let time = 1668268800000
let utcDate = new Date(time)
localTime = time - utcDate.getTimezoneOffset() * 60 * 1000
localDate = new Date(localTime)
localIso = localDate.toISOString()
console.log(utcDate.toISOString(), localIso)

Answer 2

That's correct.

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/toISOString

The timezone is always zero UTC offset, as denoted by the suffix Z.

Answer 3

You can create an ISO time string using the Date functions: Date.getFullYear(), Date.getMonth(), Date.getDate() etc.

Once we have each of these values, we can combine to create the desired ISO string, adding the UTC offset in the format [+-]HH:MM.

This date will be formatted using the local timezone of the client.

function formatUTCOffset(minutes) {
    return (minutes >= 0 ? '+': '-') + (Math.floor(Math.abs(minutes) / 60) + '').padStart(2, '0') + ':' + (Math.abs(minutes) % 60  + '').padStart(2, '0');
}

function formatDateElements(separator, ...elements) {
    return elements.map(x => (x + '').padStart(2, '0')).join(separator);
}

function formatLocalISO(date) {
    let isoDate = formatDateElements('-', date.getFullYear(), date.getMonth() + 1, date.getDate());
    isoDate += 'T' + formatDateElements(':', date.getHours(), date.getMinutes(), date.getSeconds());
    return isoDate + formatUTCOffset(-date.getTimezoneOffset());
}

let time = 1668268800000
console.log('ISO (1668268800000):', formatLocalISO(new Date(time)))
console.log('ISO (now):          ', formatLocalISO(new Date()))
    

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