tz.js/xtz.js

var XTZ;

(function () {
    "use strict";

    function toTimeZone(date, timeZone) {
        // ISO string or existing date object
        date = new Date(date);
        var options = {
            timeZone: timeZone,
            year: "numeric",
            month: "numeric",
            day: "numeric",
            hour12: false,
            hour: "numeric",
            minute: "numeric",
            second: "numeric",
            fractionalSecondDigits: 3,
        };

        var tzOptions = Object.assign({ timeZoneName: "long" }, options);

        // Every country uses the same year and months, right?
        var formater = new Intl.DateTimeFormat("default", tzOptions);
        var parts = formater.formatToParts(date);

        var whole = {};
        parts.forEach(function (part) {
            var val = part.value;
            switch (part.type) {
                case "literal":
                    // ignore separators and whitespace characters
                    return;
                case "timeZoneName":
                    // keep as is - it's a string
                    break;
                case "month":
                    // months are 0-indexed for new Date()
                    val = parseInt(val, 10) - 1;
                    break;
                case "hour":
                    // because sometimes 24 is used instead of 0, make 24 0
                    val = parseInt(val, 10) % 24;
                    break;
                case "fractionalSecond":
                    // fractionalSecond is a dumb name - should be millisecond
                    whole.millisecond = parseInt(val, 10);
                    return;
                default:
                    val = parseInt(val, 10);
            }
            // whole.month = 0;
            whole[part.type] = val;
        });

        whole.timeZone = timeZone;
        whole.offset = getOffset(date, whole);
        whole.toISOString = _toOffsetISOString;
        return whole;
    }

    function toTimeZoneISOString(date, timeZone) {
        var whole = toTimeZone(date, timeZone);
        return toOffsetISOString(whole);
    }

    function _toOffsetISOString() {
        return toOffsetISOString(this);
    }

    function getOffset(utcDate, tzD2) {
        var tzDate = new Date(toOffsetISOString(tzD2));
        var diff =
            Math.round(tzDate.valueOf() - utcDate.valueOf()) / (60 * 1000);
        return diff;
    }

    function p2(x) {
        return String(x).padStart(2, "0");
    }

    function p3(x) {
        return String(x).padStart(3, "0");
    }

    function formatOffset(minutes) {
        if (!minutes) {
            return "Z";
        }

        var h = Math.floor(Math.abs(minutes) / 60);
        var m = Math.abs(minutes) % 60;
        var offset = "";
        if (minutes > 0) {
            offset = "+";
        } else if (minutes < 0) {
            offset = "-";
        }

        // +0500, -0730
        return (
            offset +
            h.toString().padStart(2, "0") +
            m.toString().padStart(2, "0")
        );
    }

    function toOffsetISOString(d) {
        var offset = formatOffset(d.offset);
        return (
            `${d.year}-${p2(d.month + 1)}-${p2(d.day)}` +
            `T${p2(d.hour)}:${p2(d.minute)}:${p2(d.second)}.${p3(
                d.millisecond
            )}${offset}`
        );
    }

    function toUTC(dt, tz) {
        if ("string" === typeof dt) {
            // Either of these formats should work:
            // 2021-03-14 01:15:59
            // 2021-03-14T01:15:59Z
            dt = dt
                .replace("T", " ")
                .replace("Z", "")
                .replace(" ", "T")
                .replace(/$/, "Z");
        }
        var utcDate = new Date(dt);
        var tzD2 = toTimeZone(utcDate, tz);
        var offset = tzD2.offset;
        tzD2.offset = "";

        var deltaDate = new Date(utcDate);
        deltaDate.setUTCMinutes(deltaDate.getUTCMinutes() - offset);
        var tzD3 = toTimeZone(deltaDate, tz);

        if (
            tzD3.hour === utcDate.getUTCHours() &&
            tzD3.minute === utcDate.getUTCMinutes()
        ) {
            return tzD3;
        }

        var diff = tzD3.offset - offset;
        var h = Math.floor(Math.abs(diff) / 60);
        var m = Math.abs(diff) % 60;
        var sign = Math.abs(diff) / diff;
        tzD3.hour -= h * sign;
        tzD3.minute -= m * sign;

        return tzD3;
    }

    function toUTCISOString(date, timeZone) {
        var whole = toUTC(date, timeZone);
        return toOffsetISOString(whole);
    }

    XTZ = {
        // bespoke date =>
        // 2021-11-07T3:15:59-0500
        toOffsetISOString: toOffsetISOString,

        // -240 => -0400
        formatOffset: formatOffset,

        // [ "2021-11-07T08:15:59Z", "America/New_York" ]
        // => "2021-11-07T03:15:59-0500" // 2021-11-07 03:15:59
        toTimeZone: toTimeZone,
        toTimeZoneISOString: toTimeZoneISOString,

        // [ "2021-11-07 03:15:59", "America/New_York" ]
        // => "2021-11-07T03:15:59-0500" // 2021-11-07T08:15:59Z
        toUTC: toUTC,
        toUTCISOString: toUTCISOString,
    };

    if ("undefined" != typeof module && module.exports) {
        module.exports = XTZ;
    }
}());