Listing of FormatAge.js
//------------------------------------------------------------------
//  formatAge: format the age as a text string
//  ageFormat:
//      0   years, months, and days
//      1   months and days
//      2   days only
//------------------------------------------------------------------
var monthDays = new Array (31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31);

function formatAge(name, birthdate, ageFormat) {
    var date1 = new Date(birthdate);
    var result = name + " is " + formatDelta(date1, "", ageFormat) + " old.";
    return result;
}
    
//------------------------------------------------------------------
//  formatDelta: figures the difference from d1 to d2 = "mm/dd/yyyy"
//  if either date is blank, use today
//------------------------------------------------------------------
function formatDelta(d1, d2, ageFormat) {
    //--------------------------------------------------------------
    //  compute today - birthday
    //--------------------------------------------------------------
    var date1, date2;
    if (d1 == "") date1 = new Date();
    else          date1 = new Date(d1);

    if (d2 == "") date2 = new Date();
    else          date2 = new Date(d2);
    
    //--------------------------------------------------------------
    //  swap order if necessary
    //--------------------------------------------------------------
    if (date2 < date1) {
        var datex = date1;
        date1 = date2;
        date2 = datex;
    }
    
    var deltaY = date2.getFullYear() - date1.getFullYear();
    var deltaM = date2.getMonth()    - date1.getMonth();
    var deltaD = date2.getDate()     - date1.getDate();
    
    //--------------------------------------------------------------
    //  if the day difference is negative, adjust M and D
    //--------------------------------------------------------------
    if (deltaD < 0) {
        
        var iPrev = date2.getMonth() - 1 ;      // previous month
        if (iPrev == -1) iPrev = 11;            // January - 1 = December
        deltaD = date2.getDate() + monthDays[iPrev] - date1.getDate();
        
        //----------------------------------------------------------
        // adjust for leap years
        //----------------------------------------------------------
        if (iPrev == 1 && isLeapYear(date2)) deltaD ++; 
        
        deltaM --;
    }
    
    //--------------------------------------------------------------
    //  similar fix for month difference negative
    //--------------------------------------------------------------
    if (deltaM < 0) {
        deltaY --;
        deltaM += 12;
    }
    
    //--------------------------------------------------------------
    //  format the age text (ymd is the default).
    //--------------------------------------------------------------
    var result = "";
    if (ageFormat == 0 && deltaY == 0) 
        ageFormat = 1;
        
    if (ageFormat == 1 && deltaM == 0)
        ageFormat = 2;
        
    switch (ageFormat) {
        case 1:
            deltaM += 12 * deltaY;
            result = formatPlural(deltaM, "month") + " and " + 
                     formatPlural(deltaD, "day");
            break;
        case 2:                 // all days is the hard case
            result = formatPlural(dayOfEra(date2) - dayOfEra(date1), "day"); 
            break;
        default:
            result = formatPlural(deltaY, "year") + ", " + 
                     formatPlural(deltaM, "month") + ", and " + 
                     formatPlural(deltaD, "day");
            break;
    }
    return result;
}

//------------------------------------------------------------------
//  formatPlural: add number to word, possibly add s
//------------------------------------------------------------------
function formatPlural(n, word) {
    var result = n + " " + word;
    if (n != 1) 
        result += "s";
    return result;
}

//------------------------------------------------------------------
//  dayOfEra: number of days from 1/1/0 to 1/1/yyyy
//------------------------------------------------------------------
function dayOfEra(date) {
    //--------------------------------------------------------------
    //  first get number of days from 1/1/0 to 1/1/yyyy
    //--------------------------------------------------------------
    var y = date.getFullYear() - 1;             // history
    var n = 365 * y
        + Math.floor(y / 4)
        - Math.floor(y / 400);

    //--------------------------------------------------------------
    //  Then add up the days for the current year
    //--------------------------------------------------------------
    n += date.getDate();
    
    for (var i=0; i<date.getMonth(); i++) {
        n += monthDays[i];
        if (i == 1 && isLeapYear(date)) n++;
    }
    return n;
}

//------------------------------------------------------------------
//  Is the given year a leap year?
//------------------------------------------------------------------
function isLeapYear(date) {
    var result = false;
    if (date.getFullYear() % 4 == 0 &&
        date.getFullYear() % 400 != 0 ) result = true;
    return result;
}