Question

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--;
    }
    document.write(age);

}

getAge("01/20/2011")

This shows me 0 years, but i would like to show 10 Months and 9 Months until he his birthday comes 10/20/2011.

Answer 1

It often happens that you want to know the age a person would be on a specific date,
or the years, months and days between two dates.

If you do want the age as-of today, pass a single date or datestring argument.

function getAge(fromdate, todate){
    if(todate) todate= new Date(todate);
    else todate= new Date();

    var age= [], fromdate= new Date(fromdate),
    y= [todate.getFullYear(), fromdate.getFullYear()],
    ydiff= y[0]-y[1],
    m= [todate.getMonth(), fromdate.getMonth()],
    mdiff= m[0]-m[1],
    d= [todate.getDate(), fromdate.getDate()],
    ddiff= d[0]-d[1];

    if(mdiff < 0 || (mdiff=== 0 && ddiff<0))--ydiff;
    if(mdiff<0) mdiff+= 12;
    if(ddiff<0){
        fromdate.setMonth(m[1]+1, 0);
        ddiff= fromdate.getDate()-d[1]+d[0];
        --mdiff;
    }
    if(ydiff> 0) age.push(ydiff+ ' year'+(ydiff> 1? 's ':' '));
    if(mdiff> 0) age.push(mdiff+ ' month'+(mdiff> 1? 's':''));
    if(ddiff> 0) age.push(ddiff+ ' day'+(ddiff> 1? 's':''));
    if(age.length>1) age.splice(age.length-1,0,' and ');    
    return age.join('');
}


getAge("1/25/1974")>> 37 years 8 months and 26 days

getAge("9/15/1984")>> 27 years 1 month and 5 days

getAge("12/20/1984","10,20,2011")>>26 years  and 9 months

getAge(new Date(),"12/25/2011")+' till Christmas'>>
2 months and 5 days till Christmas

getAge("6/25/2011")>> 3 months and 25 days