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Python Program to Calculate Age from a Date of Birth

Beginner · Dates and times

This program works out a person's age in years from their date of birth. Subtracting the years is not enough on its own: someone born in December has not had this year's birthday yet in October, so the program also checks whether the birthday has come.

Program

calculate_age.py
from datetime import date
birth_date = date(2008, 12, 5) # or: birth_date = date(int(input("Year: ")), int(input("Month: ")), int(input("Day: ")))
today = date(2026, 10, 18) # or: today = date.today()
age = today.year - birth_date.year
# Take off one year if this year's birthday has not happened yet
if (today.month, today.day) < (birth_date.month, birth_date.day):
age -= 1
print("Date of birth:", birth_date.strftime("%d %B %Y"))
print("Today:", today.strftime("%d %B %Y"))
print("Age:", age, "years")
Output
Date of birth: 05 December 2008
Today: 18 October 2026
Age: 17 years

How it works

  1. date(2008, 12, 5) makes a date object for 5 December 2008, written as year, month, day.
  2. The first guess at the age is today.year - birth_date.year, which is 18 here.
  3. Tuples are compared item by item, so (today.month, today.day) < (birth_date.month, birth_date.day) is True when this year's birthday is still to come.
  4. In that case one year is taken off, giving 17, because the 18th birthday is not until 5 December.
  5. The program uses a fixed "today" so its output never changes; use date.today() from the comment to work from the real date.

Other ways to write it

Age in years, months and days

Age in years, months and days
from datetime import date, timedelta
birth = date(2008, 12, 5)
today = date(2026, 10, 18) # or: today = date.today()
years = today.year - birth.year
months = today.month - birth.month
days = today.day - birth.day
if days < 0: # borrow the days of last month
months -= 1
last_month_end = today.replace(day=1) - timedelta(days=1)
days += last_month_end.day
if months < 0: # borrow 12 months from the years
years -= 1
months += 12
print(f"Age: {years} years, {months} months and {days} days")
Output
Age: 17 years, 10 months and 13 days

This works like subtraction with borrowing: when the days or the months come out negative, one month or one year is borrowed from the next column.

Days lived and the next birthday

Days lived and the next birthday
from datetime import date
birth = date(2008, 12, 5)
today = date(2026, 10, 18) # or: today = date.today()
print("Days lived:", (today - birth).days)
next_birthday = birth.replace(year=today.year)
if next_birthday < today:
next_birthday = next_birthday.replace(year=today.year + 1)
print("Next birthday:", next_birthday.strftime("%d %B %Y"))
print("Days to go:", (next_birthday - today).days)
Output
Days lived: 6526
Next birthday: 05 December 2026
Days to go: 48

Subtracting two dates gives a timedelta, whose .days is the gap in days. replace(year=...) moves the birthday into this year; a 29 February birthday needs extra care, since most years have no such date.

Try it yourself

Change the date of birth to date(2008, 10, 18) to check that someone whose birthday is today gets the new age.

Edit the program above and press Run, or open it in the full editor to save it and share it. To watch it run line by line, open it in the visualizer.

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