Stop miscalculating age in JavaScript: leap years, Feb 29, and the Jan 31 trap

Source: Dev.to

Most age calculators get it wrong

• They simply do nowYear - dobYear and forget to check if the birthday has already happened.
• They treat all months as having the same length.
• They explode on Feb 29 birthdays.
• They hit JavaScript’s “Jan 31 + 1 month = March” surprise.

If you need an age calculator you can ship, you have to handle these edge cases correctly.

What we’re actually trying to compute

Given

• dob – date of birth
• asOf – the date you want to measure age on (defaults to today)

We want

• years – full birthdays completed
• months – full months since the last birthday
• days – remaining days since that month anchor

If asOf = dob);

// ---- Years ----
let years = asOf.getFullYear() - dob.getFullYear();
const bdayThisYear = birthdayInYear(dob, asOf.getFullYear(), "FEB_28");
if (asOf  {
  it("handles birthday not yet happened this year", () => {
    const dob = new Date("2000-10-20");
    const asOf = new Date("2026-02-09");
    const r = calculateAge(dob, asOf);
    expect(r.years).toBe(25);
  });

  it("handles month‑end clamping (Jan 31)", () => {
    const dob = new Date("2000-01-31");
    const asOf = new Date("2000-03-01");
    const r = calculateAge(dob, asOf);
    // If your month add is buggy, this often breaks.
    expect(r.years).toBe(0);
    expect(r.months).toBeGreaterThanOrEqual(1);
  });

  it("handles Feb 29 birthdays with FEB_28 rule", () => {
    const dob = new Date("2004-02-29");
    const asOf = new Date("2025-02-28");
    const r = calculateAge(dob, asOf);
    // Under FEB_28 policy, birthday is considered reached on Feb 28.
    expect(r.years).toBe(21);
  });

  it("rejects asOf before dob", () => {
    const dob = new Date("2020-01-01");
    const asOf = new Date("2019-12-31");
    expect(() => calculateAge(dob, asOf)).toThrow();
  });
});

Additional edge cases you can add

• dob = 1999-12-31, asOf = 2000-01-01
• dob = 2000-02-28, asOf = 2001-02-28
• dob = 2000-03-31, asOf = 2000-04-30

The takeaway

To get correct age output:

1. Normalize to date‑only (midnight).
2. Define a consistent Feb 29 policy.
3. Clamp month ends when stepping through months.
4. Ship tests that cover the weird dates.

That’s it—no external libraries required.

Demo (optional):