How to Calculate Age in Elixir (Date.diff and Birthday Logic Examples)

If you only need a person's age in completed years, the core job is simple: subtract the birth year, then reduce by one if the birthday has not happened yet this year. In Elixir, Date already gives you the pieces you need.

Fast option: use an online age calculator

If you need an exact result in years, months, and days, or want to test your code against a known answer, start with a tool.

Open the age calculator and enter the date of birth plus any reference date.

Age in full years with Elixir

This helper returns the number of completed years as of a given date.

defmodule Age do
  def years(%Date{} = dob, %Date{} = as_of \ Date.utc_today()) do
    years = as_of.year - dob.year

    had_birthday =
      {as_of.month, as_of.day} >= {dob.month, dob.day}

    if had_birthday, do: years, else: years - 1
  end
end

Age.years(~D[1998-10-12])

Using Date.diff for a rough day check

Date.diff/2 is handy when you want to confirm the range between dates, but for age in years you should still keep the birthday check. Dividing days by 365 is only an approximation.

days = Date.diff(Date.utc_today(), ~D[1998-10-12])

Parse ISO dates safely

If the date of birth comes from a form or API, parse it first and handle invalid input explicitly.

case Date.from_iso8601("1998-10-12") do
  {:ok, dob} -> Age.years(dob)
  {:error, _} -> {:error, :invalid_date}
end

FAQ

What is the usual definition of age?

In most apps, age means completed years. Someone turns 30 on their birthday, not before.

How do I get years, months, and days?

You can build that logic manually, but month lengths and leap years make it easy to get wrong. A calculator is useful for validation.