function calcTime(city, offset) {

    // create Date object for current location
    d = new Date();
    
    // convert to msec
    // add local time zone offset 
    // get UTC time in msec
    utc = d.getTime() + (d.getTimezoneOffset() * 60000);
    
    // create new Date object for different city
    // using supplied offset
    nd = new Date(utc + (3600000*offset));
    
    // return time as a string
    return "The local time in " + city + " is " + nd.toLocaleString();

}

// 
alert(calcTime('Bangkok', '+7'));

// get Singapore time
alert(calcTime('Singapore', '+8'));

// get London time
alert(calcTime('London', '+1'));