Javascript calculate age function

getAge.js

/**
 * Tells you the age from a string date
 * 
 * @param dateString Date in string format ex: "1981/3/27"
 * 
 * @return integer The calculated age
 **/
function getAge( dateString ) {
    var today = new Date();
    var birthDate = new Date(dateString);
    var age = today.getFullYear() - birthDate.getFullYear();
    var m = today.getMonth() - birthDate.getMonth();
    if (m < 0 || (m === 0 && today.getDate() < birthDate.getDate())) {
        age--;
    }
    return age;
}