How the maths works

Dates are day numbers

Every plain date is converted to a whole number of days since 1 January 1970 using Howard Hinnant's civil-date algorithms, in the proleptic Gregorian calendar, for years 1 to 9999. Adding days and counting between dates is then integer arithmetic. No JavaScript Date objects are used for plain dates, so a time zone or daylight-saving change can never shift a result by a day.

Months and years clamp at month ends

Adding months keeps the day number and clamps to the last day of shorter months (31 January + 1 month = 28 or 29 February). Years and months are applied before weeks and days. Differences count a month once the same day number is reached. These match the ISO 8601-based Temporal proposal for JavaScript, which the test suite checks against for 20,000 random dates.

Weeks follow ISO 8601

Weeks start on Monday; week 1 contains the year's first Thursday. Week-years can differ from calendar years around New Year.

Time zones use your browser's IANA database

The time duration calculator uses the Temporal API (with the temporal-polyfill library where browsers don't yet ship it) on top of the IANA time zone database built into your browser. Times skipped by a daylight-saving change are moved forward; repeated times use the first occurrence, the same "compatible" behaviour as calendar apps.

Holiday sources

• US federal holidays: the 11 holidays in 5 U.S.C. § 6103 (Juneteenth from 2021, Martin Luther King Jr. Day from 1986), observed on the Friday before or Monday after a weekend date as published by the US Office of Personnel Management, including 31 December when 1 January falls on a Saturday.
• UK bank holidays (England and Wales): the Banking and Financial Dealings Act 1971 schedule with substitute days, Easter from the Gregorian computus, and one-off proclamations (1995 and 2020 VE Day moves, 1999 Millennium, 2002 and 2012 jubilees, 2011 royal wedding, 2022 Platinum Jubilee and State Funeral, 2023 Coronation), checked against GOV.UK. Future one-off holidays are added when proclaimed.

The plain-English box

On the home page you can type a question such as "45 working days after 3 November". An AI model (OpenAI GPT-6 Luna via Vercel AI Gateway) only converts the sentence into a structured operation such as "add 45 business days to 2026-11-03, US holidays". The answer is then calculated by the same code as the calculators, and the tape shows how the question was read, so you can check it.

Tested

The date engine has an automated test suite covering leap years (including 1900, 2000 and 2100), month-end clamping, ISO week edge years, Julian Day Numbers, 29 February birthdays, holiday rules for sample years, business-day counts, and DST transitions in New York, London, Tokyo and Lord Howe Island.