get utc time from a local time of arbitrary timezone

get utc time from a local time of arbitrary timezone

by shuNaka

HTML

<head>
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment.js/2.19.1/moment.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/moment-timezone/0.5.27/moment-timezone-with-data.min.js"></script>
</head>
<div>local time:<span id="localtime">calculating</span></div>
<div>timezone: <span id="timezone">calculating</span></div>
<div>utc time: <span id="utctime">calculating</span></div>

JavaScript

// Arbitrary localtime. Suppose it is a Tokyo time.
const datetime = "2020-02-12 10:30:00";
const timezone = "Asia/Tokyo";

document.getElementById("localtime").innerHTML = datetime;
document.getElementById("timezone").innerHTML = timezone;
document.getElementById("utctime").innerHTML = getUtcFromLocaltime(datetime, timezone);

function getUtcFromLocaltime (localtimeString, timezone) {
    // Change it to moment object
    const localTime = moment(localtimeString, 'YYYY-MM-DD HH:mm:ss', true);
    // Get the timezone
    const timeDiff  = moment().tz(timezone).format('ZZ')
    // Get the time difference
    let numTimeDiff = Number(timeDiff.substr(1)) / 100;
    numTimeDiff = hourAdjust(numTimeDiff);
    // Get UTC from the timezone（+0900）
    if (timeDiff.charAt(0) === '+') {
        return localTime.subtract({hours: numTimeDiff}).format('YYYY-MM-DD HH:mm:ss');
    } else if (timeDiff.charAt(0) === '-') {
        return localTime.add({hours: numTimeDiff}).format('YYYY-MM-DD HH:mm:ss');
    }
}


function hourAdjust (hour) {
    let mhour = Math.floor(hour);
    let min = (Math.floor(hour * 100) % 100) / 6;
    return Number(mhour+"."+min);
}