get_timezone_offset.js

export function getTimezoneOffset(date, timeZone) {
    const parts = new Intl.DateTimeFormat('en-US', {
        timeZone,
        hour12: false,
        year: 'numeric', month: 'numeric', day: 'numeric',
        hour: 'numeric', minute: 'numeric', second: 'numeric'
    }).format(date).split(/[\/\s:]/);
    // The year string contains a comma, like "2016,", so we strip it using parseInt
    const year = parseInt(parts[2], 10);
    // The month is rendered in human readable terms, which are betweeen 1-12, but the API requires 0-11
    const month = parts[0] - 1;
    const day = parts[1];
    const hour = parts[3];
    const minute = parts[4];
    const second = parts[5];

    const t1 = Date.UTC(year, month, day, hour, minute, second);
    const t2 = new Date(date).setMilliseconds(0);

    return (t1 - t2) / 60000;
}