using javascript to calculate the age

Question

I want to calculate age of anything.i already wrote the program. I took a date object and assigned value into it $birth_date=new Date('Dec,15,1992'); then i subtract the value with the current data.According to the subtraction it should returns 23 years 0 months 14 days.but it returns 23 years 4 months 14 days.years and days are OK but the month misleads.

would you pls tell me why showing this weird results?

following code is HTML and next to Javascript

$(document).ready(function(){
var birth_date = new Date('Dec, 15, 1992').getTime();

var years,months,days, hours, minutes, seconds;
var ageCount = document.getElementById('counter');
setInterval(function(){

 var current_date = new Date().getTime();
 var total_sec =(current_date-birth_date) / 1000;
 years=parseInt(total_sec/(86400*30*12));
 var second_left=total_sec%(86400*30*12);
 months=parseInt(second_left/(86400*30));
 second_left=second_left%(86400*30);
 days=parseInt(second_left/86400);
 second_left=second_left%(86400);
 hours=parseInt(second_left/3600);
 second_left=second_left%3600;
 minutes=parseInt(second_left/60);
 seconds=parseInt(second_left%60);


  ageCount.innerHTML=years+' Years '+months+' Months '+days+' Days '+hours+
 ' Hours '+minutes+' Minutes '+seconds+' Seconds';

},500);

});

<div id="counter">
 
</div>
<script src="http://code.jquery.com/jquery-1.11.0.min.js"></script>

Answer 1

I think it would make more sense to just use the basic notations as listed here for getting metrics such as date , hour , minutes, seconds from the date difference

$(document).ready(function(){
var birth_date = new Date('Feb, 15, 1992');

var years,months,days, hours, minutes, seconds;
var ageCount = document.getElementById('counter');
setInterval(function(){

var current_date = new Date();
var YearDiff =  (current_date.getYear() - birth_date.getYear());
var monthDiff = (current_date.getMonth() - birth_date.getMonth());
var daysDiff = (current_date.getDate() - birth_date.getDate());
var hoursDiff = (current_date.getHours() - birth_date.getHours());
var minDiff = (current_date.getMinutes() - birth_date.getMinutes());
var secDiff = (current_date.getSeconds() - birth_date.getSeconds());


     ageCount.innerHTML=YearDiff+' Years '+monthDiff+' Months '+daysDiff+' Days '+hoursDiff+
    ' Hours '+minDiff+' Minutes '+secDiff+' Seconds';

},500);

});

https://jsfiddle.net/DinoMyte/138bhovx/2/

Answer 2

You Can simply make this calculation with the following implementation.

https://jsfiddle.net/wqm704xm/1/

var birthday = new Date('Dec,15,1992');
var today;
var $dateEl = document.getElementById('date');

setInterval(writeValue, 100);

function writeValue() {
   today = new Date();
   var calculatedDate = calculate(normalizeDate(birthday), normalizeDate(today));

   $dateEl.innerHTML = 
    'years: ' + calculatedDate.year + '<br>' +
    'months: ' + calculatedDate.month + '<br>' +
    'days: ' + calculatedDate.day + '<br>' +
    'hours: ' + calculatedDate.hours + '<br>' +
    'minutes: ' + calculatedDate.minutes + '<br>' +
    'seconds: ' + calculatedDate.seconds + '<br>' 
  ;
}
  

function calculate(firstDate, currentDate) {
     return {
         year: currentDate.year - firstDate.year,
         month: currentDate.month - firstDate.month,
         day: currentDate.day - firstDate.day,
         hours: currentDate.hours - firstDate.hours,
         minutes: currentDate.minutes - firstDate.minutes,
         seconds: currentDate.seconds - firstDate.seconds
     }
}

function normalizeDate(date) {
  return {
    year: date.getFullYear(),
    month: date.getMonth() + 1,
    day: date.getDate(),
    hours: date.getHours(),
    minutes: date.getMinutes(),
    seconds: date.getSeconds()
  };
}

<div id="date"></div>

Answer 3

Months is incorrect because you are assuming that every month is only 30 days long, when months can be 28, 29, 30 or 31 days in length (and in some circumstances, other lengths).

To quickly see where you are going wrong, and ignoring leap years, 30/days a month would mean 30*12 only 360 days per year. That means every year you miss 5 days. So given 23 years, that's roughly 23*5 days difference, or ~4 months.

There were 5 leap years in between your date and now, adding 5 more days to the difference. This completely accounts for your 4 months (120 days).

You need to adjust your algorithm to account for these differences.

Additional info:

If you are using this in production code and require accurate dates - please keep in mind a date is not sufficient information to calculate precise differences in terms of number of days. You'll also need information such as time zone and location and handle all sorts of lovely time discontinuations.

For example: In 2011 Samoa jumped to the other side of the international date line. So 29-Dec-2011 to 1-Jan-2012 is 1 day longer in Australia than it was in Samoa.

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