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function gmmktime() {
    // Get UNIX timestamp for a GMT date  
    // 
    // version: 1008.1718
    // discuss at: http://phpjs.org/functions/gmmktime    // +   original by: Brett Zamir (http://brett-zamir.me)
    // +   derived from: mktime
    // *     example 1: gmmktime(14, 10, 2, 2, 1, 2008);
    // *     returns 1: 1201875002
    // *     example 2: gmmktime(0, 0, -1, 1, 1, 1970);    // *     returns 2: -1
    var d = new Date(), r = arguments, i = 0,
        e = ['Hours', 'Minutes', 'Seconds', 'Month', 'Date', 'FullYear'];
 
    for (i = 0; i < e.length; i++) {        if (typeof r[i] === 'undefined') {
            r[i] = d['getUTC' + e[i]]();
            r[i] += (i === 3); // +1 to fix JS months.
        } else {
            r[i] = parseInt(r[i], 10);            if (isNaN(r[i])) {
                return false;
            }
        }
    }    
    // Map years 0-69 to 2000-2069 and years 70-100 to 1970-2000.
    r[5] += (r[5] >= 0 ? (r[5] <= 69 ? 2e3 : (r[5] <= 100 ? 1900 : 0)) : 0);
    
    // Set year, month (-1 to fix JS months), and date.    // !This must come before the call to setHours!
    d.setUTCFullYear(r[5], r[3] - 1, r[4]);
    
    // Set hours, minutes, and seconds.
    d.setUTCHours(r[0], r[1], r[2]); 
    // Divide milliseconds by 1000 to return seconds and drop decimal.
    // Add 1 second if negative or it'll be off from PHP by 1 second.
    return (d.getTime() / 1e3 >> 0) - (d.getTime() < 0);
}

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gmmktime(14, 10, 2, 2, 1, 2008);

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1201875002

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gmmktime(0, 0, -1, 1, 1, 1970);

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-1