Calculate Age Accurately: Edge Cases, Time Zones, and Clear Rules

Age seems simple until it isn’t. Leap years, time zones, and end‑of‑month rollovers can all cause off‑by‑one errors. This checklist keeps calculations consistent and defensible.

Pitfalls to Watch

• Time zones: Midnight crossings can change the calendar date—normalize to local midnight for both dates.
• Leap day births (Feb 29): Choose a consistent rule for non‑leap years (Feb 28 or Mar 1) and document it.
• Month‑end rollovers: Adding months across short months (e.g., Jan 31 → Feb) needs a defined policy.
• Inclusive/exclusive: Clarify whether age increments at the start of the birthday.

Recommended Approach

1. Convert inputs to the same local time zone and strip times (set to 00:00).
2. Compute year difference; if the current month/day precedes the birth month/day, subtract one year.
3. For months/days, use a calendar function that accounts for varying month lengths.

Testing Tricky Cases

• Feb 29 to non‑leap years
• End‑of‑month births and adds (Jan 31, Apr 30)
• Cross‑DST transitions (shouldn’t affect date math, but test anyway)

Tools

• Use battle‑tested date libraries that handle calendar math and locales.
• Document your conventions in code (comments and tests) and user docs.

FAQs

How to handle Feb 29 birthdays? Many systems use Feb 28 in non‑leap years; some use Mar 1. Pick one, apply it consistently, and communicate it where relevant.