same as Date.getTimezoneOffset() but with DST

getTimeZoneOffsetDST.js

/**
 * Calculate the time zone offset with DST !
 * e.g. Prague - if DST enabled ? -60 (summer) : -120 (winter)
 *
 * NOTE: this is to be compatible to how Rails helper
 * time_zone_options_for_select renders zones e.g. :
 * - in summer (GMT+01:00) Prague
 * - in winter (GMT+02:00) Prague
 */
function getTimeZoneOffsetDST() {
    // NOTE: return new Date().getTimezoneOffset() is not enought !
    var today = new Date();
    // 2nd Sunday in March can't occur after the 14th :
    var dstBeg = new Date("March 14, "+ today.getFullYear() +" 02:00:00");
    // 1st Sunday in November can't occur after the 7th :
    var dstEnd = new Date("November 07, "+ today.getFullYear() +" 02:00:00");
    dstBeg.setDate( 14 - dstBeg.getDay() ); // Calculate second Sunday in March
    dstEnd.setDate( 7 - dstEnd.getDay() ); // Calculate first Sunday in November
    if ( today >= dstBeg && today < dstEnd ) { // isDST
        // e.g. for GMT+02:00 returns -120 !
        return today.getTimezoneOffset() + 60;
    }
    else {
        return today.getTimezoneOffset();
    }
}