Date Calculations

Age Calculator

Calculate your exact age in years, months, and days with our precise age calculator. Find out your age at any specific date, calculate birth date from age, and discover how many days you've lived. Perfect for determining age requirements, planning birthday celebrations, or calculating age-related milestones. A comprehensive tool for age verification and birth date calculations.

Date Calculator

An advanced tool for calculating the difference between two dates in days, months, and years. The date calculator allows you to easily determine the duration between past or future dates. Perfect for birthdays, anniversaries, special occasions, project deadlines, and business day calculations.

Add/Subtract Days Calculator

Calculate a new date by adding or subtracting days, weeks, months, or years from a specific date. Perfect for determining appointment dates, due dates, deadline calculations, and more. A versatile tool for forward and backward date calculations.

Date and Time Calculator

Calculate the difference between two dates and times. See results in years, months, days, hours and minutes.

Choosing the right date calculator

Four different questions get asked about dates, and each needs different arithmetic. Picking the wrong tool produces an answer that looks plausible and is quietly off by a day. This page maps the question you have onto the calculator that answers it.

Match the question to the tool

Start from what you are holding and what you are missing.

  • You have two dates and want the gap: use the date difference calculator. It returns days, weeks, months, and years between them.
  • You have one date and a period, and want the resulting date: use the add or subtract days calculator. It handles month-end clamping correctly.
  • You have a birth date and want completed years: use the age calculator, which applies the borrow rules that plain subtraction gets wrong.
  • You have two moments including clock time: use the date and time calculator, which reports hours and minutes alongside days.

The three rules that cause most errors

Almost every wrong date answer traces back to one of these, regardless of which tool produced it.

  1. Inclusive versus exclusive counting. 1 March to 5 March is 4 days elapsed but 5 dates involved. Decide which you mean before you count.
  2. Months are not fixed lengths. "Three months" and "90 days" differ in almost every calendar window, and contracts that mix the two invite disputes.
  3. Month-end clamping is not reversible. 31 March minus one month is 28 February, but 28 February plus one month is 28 March, not 31 March.

Leap year rules in full

The Gregorian rule has three clauses, and most people only remember the first. A year is a leap year if it is divisible by 4, except that century years are not, unless they are divisible by 400.

So 1996 and 2024 are leap years, 1900 and 2100 are not, and 2000 was. The rule gives an average year of 365.2425 days against a true tropical year of about 365.24219 days, an error of roughly one day every 3,200 years.

ISO week numbers behave differently from months

ISO 8601 weeks start on Monday, and week 1 is the week containing the first Thursday of January. A year therefore has 52 or 53 ISO weeks, and early January can belong to the previous ISO year.

1 January 2027 is a Friday, which places it in ISO week 53 of 2026. Reporting systems that group by ISO week and by calendar month will not reconcile, and the discrepancy always shows up in the first and last weeks of a year.

Frequently Asked Questions

Use the add or subtract days calculator with the period exactly as the contract states it. If the contract says days, use days. If it says months, use months. Converting between them yourself is where deadlines get missed.

Business-day counting needs a specific holiday calendar. Those differ by country, by region within a country, and sometimes by sector, so a single generic answer would be wrong for most users.

365.2425 under the Gregorian rule. Financial conventions often use 360 or 365 instead, and which one applies changes the interest owed, so loan documents state the day-count basis explicitly.

The arithmetic assumes a proleptic Gregorian calendar throughout. Historical records before Gregorian adoption used the Julian calendar, and adoption spanned 1582 in Catholic Europe to 1923 in Greece, so expect a 10 to 13 day discrepancy.

Related calculators

References

Last reviewed: 2026-08-07. This page is informational. For legal, medical, tax, or financial decisions, confirm the result with a qualified professional.