Age Calculation Function

Question

I'm trying to calculate the age of the user in months, days, hours and minutes including leap years.. However, rather than using the select/option in HTML, I've input the options via Javascript. Due to this, I'm rather confused on how to do the calculations in Javascript now. Could anyone point me in the right direction to do these calculations?

HTML :

`  <ul class = "mainList">
    <h1>
      Age Calculator
    </h1>
    <h2> Enter Your Birthday</h2>
  <li class = "firstList">
    <p class = "datePara">
      Date:
    </p>
    <select id = "day">
    </select>
  </li>
  <li>
    <p class = "monthPara">
      Month:
    </p>
    <select id = "month">
    </select>
  </li>
  <li>
    <p class = "yearPara">
      Year:
    </p>
    <select id = "year">
    </select>
  </li>
  </ul>`

Javascript : `

var startyear = "1950";
var endyear = "2015";
var dat = new date();
var curday = dat.getDate();
var curmon = dat.getMonth() + 1;
var curyear = dat.getFullYear();


function initForm() {

  document.getElementById('day').focus();

for (var i = 1; i <= 31; i ++) {

  var thisDay = document.getElementById('day');
  var dayOption = document.createElement('option');

  dayOption.text = i;
  thisDay.add(dayOption, i);
}

for (var i = 1; i <= 12; i ++) {

  var thisMonth = document.getElementById('month');
  var monthOption = document.createElement('option');

  monthOption.text = i;
  thisMonth.add(monthOption, i);
}

for (var i = startyear; i <= endyear; i ++) {

  var thisYear = document.getElementById('year');
  var yearOption = document.createElement('option');

  yearOption.text = i;
  thisYear.add(yearOption, i);
}
}`

Answer 1

MomentJS has a fromNow() function that will handle this automatically, taking care of leap years/local time/etc. It's pretty easy to use; you just instantiate a 'moment' from the time your user inputs and call moment.fromNow().

EDIT: Alternative with no moment:

day=document.getElementById('day').value;
month=document.getElementById('month').value;
year=document.getElementById('year').value;
birthdate=new Date(year, month-1, day); // 'month' is zero-indexed.
today = new Date();
age = (today-birthdate) / (1000*60*60*24*365.25);
// /1000 = seconds, /60=minutes, /60=hours, /24=days, /365.25=years, factor leap years

EDIT: I realize you're going to implement yourself, but here's a quick example anyway:

day=document.getElementById('day').value;
month=document.getElementById('month').value;
year=document.getElementById('year').value;
birthdate=new Date(year, month-1, day); // 'month' is zero-indexed.
today = new Date();
age = moment(today).diff(moment(birthdate), 'year');

You could skip the momentand just take the difference between the date objects (today-birthdate), which will give you the time interval in ms. The advantage of using the moment is that the time interval will be correct while factoring in leap years and the start date of the interval.

Answer 2

To calculate something like this, you'll want to start by taking the difference between the years, months, days, etc., of the two dates:

var yearsBetween = currentDate.getYear() - targetDate.getYear();
var monthsBetween = currentDate.getMonth() - targetDate.getMonth();
var daysBetween = currentDate.getDate() - targetDate.getDate();
var hoursBetween = currentDate.getHours() - targetDate.getHours();
var minutesBetween = currentDate.getMinutes() - targetDate.getMinutes();
var secondsBetween = currentDate.getSeconds() - targetDate.getSeconds();

But some of these values will be negative. In those cases, you'll need to adjust the next-highest unit value to compensate:

if(secondsBetween < 0) {
  secondsBetween += 60;
  minutesBetween -= 1;
}
if(minutesBetween < 0) {
  minutesBetween += 60;
  hoursBetween -= 1;
}
...
if(daysBetween < 0) {
  // http://stackoverflow.com/a/1184359/120955
  var daysInTargetMonth = new Date(targetDate.getYear(), targetDate.getMonth() + 1, 0).getDate();
  daysBetween += daysInTargetMonth;
  monthsBetween -= 1;
}

Then, finally, it sounds like you don't care about years, so you'll want to roll those into the number of months:

monthsBetween += yearsBetween * 12;