Get the Number of Years Between Two Dates with PHP and JavaScript
In Brief
Decide whether you need elapsed duration or complete calendar years before comparing two dates. Dividing milliseconds by 365 days gives only an approximation because leap years and calendar boundaries intervene; date‑aware comparisons are safer for ages and anniversaries. Time zones also affect the date at a boundary, so normalise the inputs to the intended calendar context first.
Working with dates is a pretty common task for web developers, especially developers working with dynamic content or content management systems. Often this just involves accepting a DateTime object and formatting it to appear a certain (more human‑readable) way. However, there can be much more advanced use cases ‑ for instance when building a blog and showing dates in time‑since format: "1 year ago", "2 years ago", etc.
This is exactly what I do here on my own website to generate my Timeline page where I use Twitter and the #todayi hashtag to offer brief insights into my working day‑to‑day. I've also toyed with the idea of changing my blog to display dates in this format.
So in this article, we are going to take a look at how to get the number of years between two dates, both via the back‑end with PHP and client‑side in JavaScript.
PHP
So, to do this in PHP let's grab today's date using the DateTime class, and figure out the years between today and the 1st of January in the year 2000:
<?php $today = new DateTime(); $millennium = new DateTime('2000-01-01'); $years_between = $millennium -> diff($today); echo $years_between -> y;?>When this article was published in 2014, this returned 14. The value naturally changes as the end date moves.
JavaScript
So let's take a look at doing this in JavaScript. It's a little trickier, and there are a lot of recommendations to just use libraries like Moment.js (which, at over 2KB, is a pretty big dependency for what should be quite a simple operation), but here's a way to do this with just vanilla JS:
function years_between(dateFrom, dateUntil) { var diff = (dateFrom.getTime() - dateUntil.getTime()) / 1000; diff /= 60 * 60 * 24; var rounded = Math.abs(diff / 365) | 0; return rounded;}var millennium = new Date('January 1 2000');var today = new Date();console.log(years_between(millennium, today));With JavaScript, the Date objects represent the number of milliseconds since 1st January 1970 UTC (known as Epoch or Unix time). Here, we're finding the difference between the two dates in milliseconds, converting it to days, dividing by 365, and rounding down to produce an elapsed‑year estimate.
This is a much longer walk to get to the same place we achieved with four lines of PHP, but still very achievable nevertheless. There are a few minor complications if you are dealing with dates before 1970 (where Date should return a negative number). Because it divides by 365, though, this JavaScript version is only an approximation: leap years and calendar boundaries can produce the wrong full‑year count. Use a calendar‑aware comparison when anniversaries, ages, or legal dates need to be exact.
Postscript
July 2026: The changing result in this example was 14 when the article was published in 2014; it was never a fixed output. The PHP DateTime::diff() example uses calendar dates, whilst the JavaScript division by 365 is only approximate. For exact modern date arithmetic, Temporal is the better model once the target runtimes support it.