Calculate age given the birth date in the format Y

Question

How can I calculate an age in years, given a birth date of format YYYYMMDD? Is it possible using the Date() function?

I am looking for a better solution than the one I am using now:

var dob = '19800810';
var year = Number(dob.substr(0, 4));
var month = Number(dob.substr(4, 2)) - 1;
var day = Number(dob.substr(6, 2));
var today = new Date();
var age = today.getFullYear() - year;
if (today.getMonth() < month || (today.getMonth() == month && today.getDate() < day)) {
  age--;
}
alert(age);

Answer 1

I have a pretty answer although it's not my code. Unfortunately I forgot the original post.

function calculateAge(y, m, d) {
    var _birth = parseInt("" + y + affixZero(m) + affixZero(d));
    var  today = new Date();
    var _today = parseInt("" + today.getFullYear() + affixZero(today.getMonth() + 1) + affixZero(today.getDate()));
    return parseInt((_today - _birth) / 10000);
}
function affixZero(int) {
    if (int < 10) int = "0" + int;
    return "" + int;
}
var age = calculateAge(1980, 4, 22);
alert(age);

Answer 2

I would go for readability:

function _calculateAge(birthday) { // birthday is a date
    var ageDifMs = Date.now() - birthday.getTime();
    var ageDate = new Date(ageDifMs); // miliseconds from epoch
    return Math.abs(ageDate.getUTCFullYear() - 1970);
}

Disclaimer: This also has precision issues, so this cannot be completely trusted either. It can be off by a few hours, on some years, or during daylight saving (depending on timezone).

Instead I would recommend using a library for this, if precision is very important. Also @Naveens post, is probably the most accurate, as it doesn't rely on the time of day.

---

Benchmarks: http://jsperf.com/birthday-calculation/15

Answer 3

I used this approach using logic instead of math. It's precise and quick. The parameters are the year, month and day of the person's birthday. It returns the person's age as an integer.

function calculateAge(year, month, day) {
        var currentDate = new Date();
        var currentYear = currentDate.getFullYear();
        var currentMonth = currentDate.getUTCMonth() + 1;
        var currentDay = currentDate.getUTCDate();
        // You need to treat the cases where the year, month or day hasn't arrived yet.
        var age = currentYear - year;
        if (currentMonth > month) {
            return age;
        } else {
            if (currentDay >= day) {
                return age;
            } else {
                age--;
                return age;
            }
        }
    }

Answer 4

Yet another solution:

/**
 * Calculate age by birth date.
 *
 * @param int birthYear Year as YYYY.
 * @param int birthMonth Month as number from 1 to 12.
 * @param int birthDay Day as number from 1 to 31.
 * @return int
 */
function getAge(birthYear, birthMonth, birthDay) {
  var today = new Date();
  var birthDate = new Date(birthYear, birthMonth-1, birthDay);
  var age = today.getFullYear() - birthDate.getFullYear();
  var m = today.getMonth() - birthDate.getMonth();
  if (m < 0 || (m === 0 && today.getDate() < birthDate.getDate())) {
    age--;
  }
  return age;
}

Answer 5

I know this is a very old thread but I wanted to put in this implementation that I wrote for finding the age which I believe is much more accurate.

var getAge = function(year,month,date){
    var today = new Date();
    var dob = new Date();
    dob.setFullYear(year);
    dob.setMonth(month-1);
    dob.setDate(date);
    var timeDiff = today.valueOf() - dob.valueOf();
    var milliInDay = 24*60*60*1000;
    var noOfDays = timeDiff / milliInDay;
    var daysInYear = 365.242;
    return  ( noOfDays / daysInYear ) ;
}

Ofcourse you could adapt this to fit in other formats of getting the parameters. Hope this helps someone looking for a better solution.

Answer 6

Important: This answer doesn't provide an 100% accurate answer, it is off by around 10-20 hours depending on the date.

There are no better solutions ( not in these answers anyway ). - naveen

I of course couldn't resist the urge to take up the challenge and make a faster and shorter birthday calculator than the current accepted solution. The main point for my solution, is that math is fast, so instead of using branching, and the date model javascript provides to calculate a solution we use the wonderful math

The answer looks like this, and runs ~65% faster than naveen's plus it's much shorter:

function calcAge(dateString) {
  var birthday = +new Date(dateString);
  return ~~((Date.now() - birthday) / (31557600000));
}

The magic number: 31557600000 is 24 * 3600 * 365.25 * 1000 Which is the length of a year, the length of a year is 365 days and 6 hours which is 0.25 day. In the end i floor the result which gives us the final age.

Here is the benchmarks: http://jsperf.com/birthday-calculation

To support OP's data format you can replace +new Date(dateString);
with +new Date(d.substr(0, 4), d.substr(4, 2)-1, d.substr(6, 2));

If you can come up with a better solution please share! :-)