DateUtils

Date Calculators and Date Reference

20 free calculators and converters for dates and times, and reference pages that explain the rules behind them.

Calculators by category

Understanding date calculations

Most date questions have more than one reasonable answer, and the difference is usually a convention rather than a mistake. These are the ones that cause most disagreements:

  • Does the first day count? From 1 March to 31 March is 30 days by subtraction, or 31 if both dates are counted. A hotel stay counts nights; a period of leave usually counts both days. See counting days between dates.
  • What is one month after 31 January? February has no 31st, so most calculators, spreadsheets included, give the last day of February. Adding months in two steps can then give a different answer from adding them in one. See adding months and years.
  • Which week is 1 January in? Under ISO 8601 it can belong to week 52 or 53 of the previous year: 1 January 2021 was in week 53 of 2020. See ISO week numbers.
  • Is a day always 24 hours? Not on the days clocks change, which are 23 or 25 hours long where daylight saving applies. That is why the gap between London and New York is 4 hours for a few weeks each year rather than 5. See time zones and daylight saving.

Every DateUtils calculator states which convention it uses and offers the alternative where both are common. The methodology page collects those rules in one place.

Date reference

Explanations that stand on their own, with tables and worked examples. See all reference pages.

How the calendar works

  • The Gregorian calendar The calendar most of the world uses for civil dates: where it came from, how it differs from the Julian calendar, and how dates before 1582 are handled.
  • Leap years The three-part Gregorian leap-year rule, why century years are different, a list of leap years, and what 29 February does to date arithmetic.
  • Days in each month A table of month lengths, the day of the year each month starts on, quarter lengths, and how to remember which months have 30 or 31 days.
  • ISO week numbers How ISO 8601 numbers weeks, why a year can have 53 weeks, why 1 January can belong to the previous year, and how ISO weeks differ from other week numbering.

Counting and date arithmetic

  • Counting days between dates Why two people can count the same date range and get different answers: inclusive and exclusive counting, calendar days versus elapsed time, and month-based spans.
  • Adding months and years What "one month after 31 January" means, why adding months in steps can give a different answer, and how spreadsheets and programming languages disagree.
  • Business days What counts as a business day, how weekends and holidays are excluded, the counting conventions behind deadlines, and how spreadsheet functions compare.

Timestamps and time zones

  • Unix time What a Unix timestamp counts, how to tell seconds from milliseconds, negative timestamps, the year 2038 problem and why leap seconds are ignored.
  • Time zones and daylight saving UTC offsets, IANA zone names, why abbreviations are ambiguous, how daylight saving creates skipped and repeated times, and why the gap between two cities changes.

About DateUtils

DateUtils gives a clear result first and the reasoning next. Each calculator page explains how its result is reached, works through examples and lists the edge cases, such as leap days, month ends and clock changes. Calculations run in your browser rather than on a server.

There are no accounts and nothing to install. Each tool has its own address, loads quickly on a phone and works with a keyboard and screen reader. Read more about DateUtils, or report a result that looks wrong.