Age Calculator

Level: Intermediate 30–60 min

Concepts: Edge CasesAlgorithms

Solutions: C# | TypeScript | Python

Calculate a person’s age at a given date. Do this by creating a single public method that takes a person’s birth date and a target date to compare to and returns their age as an integer.

Examples

Zenith was born on 28 October 2016 how old was she on 5 November 2022? Answer 6.

Edge Cases to Consider

  • Leap years and handling of Feb 29 birthdays
  • Birthdate is after the target date (future date)
  • Birthdate or target date is today
  • Time zones affecting the calculation

Bonus

In the age of Social Media birthday, weeks are a big deal. To allow for this, add a new method to the Calculator that returns the start date of the person’s birthday week, as a string, according to the following rules.

  • If my birthday is Thursday-Saturday, my birthday week starts on the Sunday of that week. E.g. If I was born on September 9, 2017, my birthday week will start on September 3, 2017.
  • If my birthday is Sunday-Wednesday, my birthday week starts 6 days back. E.g. If I was born on September 3, 2017, my birthday week will start on August 28, 2017.

Reference Walkthrough

Reference implementations in C#, TypeScript, and Python live at tddbuddy-reference-katas/age-calculator. The reference exposes the canonical primitive — calculate(birthdate, today) — with the reference date passed in explicitly rather than read from the system clock. That discipline is the teaching point: ambient-clock access turns every birthday-boundary scenario (“on her seventh birthday she is 7”) into a flake unless tests can fix “today” to a specific date. Eleven scenarios cover the TDD Buddy example, birthday-as-boundary (exact-day turn-over), day-before-birthday, same-day-as-today, the leap-day-baby triad (Feb 28 in non-leap → still the previous age, Mar 1 in non-leap → aged up, Feb 29 in leap → aged up exactly), the across-the-year-boundary newborn, and a future-birthdate guard.

  • C# (.NET 8, xUnit, FluentAssertions 6.12.0) — walkthrough — public static int Calculate(DateOnly birthdate, DateOnly today)
  • TypeScript (Node 20, Vitest 1.6, TS 5 strict) — walkthrough — UTC-disciplined Date: construct via Date.UTC(...), read via getUTCFullYear() / getUTCMonth() / getUTCDate() so calendar components stay stable across host time zones
  • Python (3.11, pytest) — walkthrough — datetime.date with native tuple comparison: (today.month, today.day) < (birthdate.month, birthdate.day) is the cleanest of the three languages for this rule

The leap-day-in-a-non-leap-year rule (a Feb 29 baby turns older on March 1, not Feb 28) falls out of the (month, day) comparison without any special case — (2, 28) < (2, 29) is still true, so the age decrement fires correctly. The future-birthdate guard throws an idiomatic exception (ArgumentException / Error / ValueError) with the byte-identical message birthdate is after today.

This kata ships in Agent Full-Bake mode at high gear (F1 tier): the algorithm is small enough to land as one commit per language, with a brief walkthrough noting there are no builders because the date type comes from the standard library and the inputs/outputs are the domain. See the repo’s Gears section for when high gear is the right call.

Solutions

You can find all language solutions here Age Calculator Solutions

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