Methods for converting dates between input type="datetime-local" and server-acceptable ISO 8610 format with timezone awareness.

timezone-awareness.js

function prefixWithZero(value, minLength = 2) {
    value = value + "";

    while (value.length < minLength) {
        value = `0${value}`;
    }

    return value;
}

function isoToLocal(isoTime) {
    const date = new Date(isoTime);
    const localTime = `${date.getFullYear()}-${prefixWithZero(date.getMonth() + 1)}-${prefixWithZero(date.getDate())}T${prefixWithZero(date.getHours())}:${prefixWithZero(date.getMinutes())}:${prefixWithZero(date.getSeconds())}.${prefixWithZero(date.getMilliseconds(), 3)}`;

    return localTime;
}

function localToIso(localTime) {
    const date = new Date(localTime);

    const isoTime = `${date.getUTCFullYear()}-${prefixWithZero(date.getUTCMonth() + 1)}-${prefixWithZero(date.getUTCDate())}T${prefixWithZero(date.getUTCHours())}:${prefixWithZero(date.getUTCMinutes())}:${prefixWithZero(date.getUTCSeconds())}.${prefixWithZero(date.getUTCMilliseconds(), 3)}+00:00`;

    return isoTime;
}

const isoTime = '2022-09-06T12:00:00.000+00:00';
const local = isoToLocal(isoTime); // 2022-09-06T14:00:00.000
const iso = localToIso(local); // 2022-09-06T12:00:00.000+00:00