Guides
Plain-English explanations of how dates and ages actually work — the conventions, the edge cases, and the mistakes that catch people out.
- How Age Is CalculatedA clear explanation of how age is calculated in years, months and days, why month-end dates cause disagreement between calculators, and which method to trust.
- Leap Year Birthdays ExplainedHow old is someone born on 29 February, when do they legally turn 18, and how often does their real birthday come round? A clear guide for leaplings.
- Age Reckoning Around the WorldEast Asian age reckoning, lunar birthdays and the Korean age reform of 2023. How the same person can be two different ages depending on the system used.
- How Many Days Old Am I?How to count exactly how many days old you are, why the total is not simply years times 365, and the day-count milestones worth marking.
- How Legal Age Thresholds WorkHow legal age thresholds are defined, why the day you attain an age can be disputed, and what to check before relying on any age calculator for a legal deadline.
- Why Your Birthday Changes Day Each YearThe 28-year cycle behind your birthday's weekday, why the gaps are 6, 11, 6 and 5 years, and how leap years and century years bend the pattern.
- How to Count a Deadline CorrectlyThe difference between calendar days, business days and clear days, why 'net 30' is not 30 working days, and how to count a deadline without getting it wrong.
- Why Zodiac Dates DisagreeWhy one chart says Aries starts on 20 March and another says the 21st, what being born on the cusp really means, and how the Chinese zodiac year differs.
Why we write these
A calculator gives you a number. It does not tell you whether it is the right number for your question, and with dates that distinction matters more than people expect. Two tools can disagree about an age in months and both be correct; a legal age threshold can turn on a single day; a day count and a month count can tell genuinely different stories about the same span.
These guides cover the conventions behind the arithmetic so you can tell which figure answers your question. Each one is written and checked by our editorial team, and updated when the underlying rules change — as they did in Korea in 2023.