date comparison

date.js

/**
 * Long version
 */
var isLeap = function(year) {
    return year % 4 == 0 && (year % 100 != 0 || year % 400 == 0);
};
var getAge = function(birthDate) {
    var now = new Date(),
        age = 0,
        // days since the birthdate
        days = Math.floor(( now.getTime() - birthDate.getTime() )/1000/60/60/24);
    // iterate the years
    for (var y = birthDate.getFullYear(), y2 = now.getFullYear(); y <= y2; y++){
        var daysInYear = isLeap(y) ? 366 : 365;
        if (days >= daysInYear){
            days -= daysInYear;
            age++;
            // increment the age only if there are available enough days for the year.
        }
    }
    return age;
};
// january 1st, 2011
var birth = new Date( 2011, 0, 1 );
if ( getAge(birth) < 21 ) {
    console.log('Under age!');
} else {
    console.log('Want a beer?');
}

/**
 * Short version (doesn't work well)
 * @deprecated
 */
var birthStamp = new Date( 2011, 0, 1 ).getTime();
if ( birthStamp < 21*365*86400*1000 ) {
    console.log('Under age!');
} else {
    console.log('Want a beer?');
}