Age calculator in Python (years, months and days)

Subtracting the birth year from this year is not enough. If the birthday has not come yet this year, the person is one year younger. The program checks that.

The program

python

from datetime import date


def age_in_years(birth, today):
    years = today.year - birth.year
    birthday_not_reached = (today.month, today.day) < (birth.month, birth.day)
    if birthday_not_reached:
        years -= 1
    return years


today = date(2026, 10, 22)  # fixed so the output matches this page; use date.today() in real use

people = {
    "Ada": date(1990, 3, 15),
    "Linus": date(1990, 12, 1),
    "Grace": date(2004, 10, 22),
    "Alan": date(2008, 2, 29),
}

for name, birth in people.items():
    print(f"{name:<6} born {birth}: {age_in_years(birth, today)} years old")

output

Ada    born 1990-03-15: 36 years old
Linus  born 1990-12-01: 35 years old
Grace  born 2004-10-22: 22 years old
Alan   born 2008-02-29: 18 years old

How it works

today.year - birth.year gives 36 for both Ada and Linus. Ada's birthday (15 March) has passed this year. Linus's birthday (1 December) has not, so he is still 35.
(today.month, today.day) < (birth.month, birth.day) compares two tuples. Python compares the months first, and the days only if the months are equal. So (10, 22) < (12, 1) is True.
Grace's birthday is today. (10, 22) < (10, 22) is False, so her new age counts from today.
Alan was born on 29 February. The tuple comparison needs no special rule: in years without 29 February, his age goes up on 1 March.

Years, months and days

For an exact age such as "36 years, 7 months and 7 days", count the whole months first. Then count the days since the last month anniversary:

python

import calendar
from datetime import date


def add_months(start, months):
    month_index = start.month - 1 + months
    year = start.year + month_index // 12
    month = month_index % 12 + 1
    last_day = calendar.monthrange(year, month)[1]
    return date(year, month, min(start.day, last_day))


def exact_age(birth, today):
    months = (today.year - birth.year) * 12 + today.month - birth.month
    if today.day < birth.day:
        months -= 1
    days = (today - add_months(birth, months)).days
    years, months = divmod(months, 12)
    return years, months, days


today = date(2026, 10, 22)  # fixed so the output matches this page; use date.today() in real use
birth = date(1990, 3, 15)

years, months, days = exact_age(birth, today)
print(f"{years} years, {months} months and {days} days")
print(f"That is {(today - birth).days:,} days in total.")

output

36 years, 7 months and 7 days
That is 13,370 days in total.

add_months() moves a date forward by whole months. min(start.day, last_day) handles short months: one month after 31 January is 28 February (or 29 in a leap year), not an error.
divmod(months, 12) splits the total months into years and the months left over.
today - birth gives a timedelta, and .days is the total number of days between the two dates. The days between dates program shows more of this.