I Tried to Calculate Age Between Two Dates… It’s More Annoying Than You Think

Helge Andresen — Sat, 25 Apr 2026 16:09:43 +0000

I needed a simple thing the other day:

“What’s the exact age between two dates?”

Not just years. I wanted the full breakdown—years, months, days.

Should be easy, right?

Yeah… not really.

The “simple” approach (that breaks quickly)

My first instinct was:

const now = new Date();
const birth = new Date("1990-01-01");

const years = now.getFullYear() - birth.getFullYear();

Done?

Not even close.

This ignores:

Whether the birthday has happened yet this year
Month differences
Day differences
Leap years

So you end up with something that’s almost right… which is the worst kind of wrong.

The real problem: dates are messy

Once you try to do it properly, you run into edge cases:

Months have different lengths
February exists (and sometimes has 29 days)
You have to “borrow” days from previous months
Timezones can mess with exact values

At that point it stops being a quick calculation and turns into… date logic hell.

A slightly better approach

Here’s a more correct (but still simplified) way to calculate age:

function calculateAge(birthDate) {
const today = new Date();
let years = today.getFullYear() - birthDate.getFullYear();
let months = today.getMonth() - birthDate.getMonth();
let days = today.getDate() - birthDate.getDate();

if (days < 0) {
months--;
const prevMonth = new Date(today.getFullYear(), today.getMonth(), 0);
days += prevMonth.getDate();
}

if (months < 0) {
years--;
months += 12;
}

return { years, months, days };
}

This handles the basics, but even here you’ll start noticing edge cases if you push it.

Why this is harder than it looks

The core issue is this:

👉 We think in calendar units (years/months/days)
👉 But time is actually continuous

So converting between those two worlds is where things get tricky.

At some point… it’s not worth reinventing

After messing with this for a bit, I realized:

I don’t actually want to maintain date math logic.

I just want the result.

So instead of handling every edge case myself, I ended up using a simple tool that already does it cleanly:

👉 https://snapagecalc.com

No ads, no weird UI—just gives you the exact breakdown instantly.

Takeaway

If you’re building something where age calculation matters:

Be careful with edge cases
Don’t trust “quick” solutions
Test with tricky dates (end of month, leap years, etc.)

And if you just need the answer?

Honestly… don’t overthink it.

Curious: how are you handling this?

If you’ve implemented your own age/date logic, I’m genuinely curious:

Did you build it from scratch?
Use a library?
Or just avoid the problem entirely?

Would love to hear how others approached it 👇

DEV Community: Helge Andresen

I Tried to Calculate Age Between Two Dates… It’s More Annoying Than You Think