How itsdately calculates — and how to check it
Every date tool claims to be accurate. This page names the exact algorithm behind each figure, cites where it comes from, and then runs a test suite in your browser, right now, comparing what the site computes against values that are independently and publicly known. Nothing here is a stored answer: the numbers in the "computed" column are worked out on your device as this page loads.
| What is being checked | Expected (known fact) | Computed here | Result |
|---|
Time zones and daylight saving
Clocks and conversions read the IANA time zone database (the "tz database") through the browser's own Intl.DateTimeFormat with a named zone such as Asia/Karachi. That is the same dataset operating systems use, so when a government changes its rules the correction arrives with your device's updates. The site never stores a fixed offset for a city, because fixed offsets go wrong twice a year.
Source: IANA Time Zone Database (iana.org/time-zones), via ECMA-402 Intl.
Sunrise, sunset and prayer times
Solar times are computed from the sun's position for your latitude, longitude and date — not looked up in a table. The implementation follows the standard NOAA solar equations derived from Jean Meeus, Astronomical Algorithms: solar declination and the equation of time give solar noon, and an hour angle at a given altitude gives each event.
The altitudes used are the conventional ones: −0.833° for sunrise and sunset (allowing for refraction and the solar disc), and −18° for Fajr and Isha. Asr is defined by shadow length rather than altitude — shadow equal to object height in the Standard (Shafi'i) convention, twice the height in the Hanafi convention — which is why both are shown. Where the sun never reaches an angle (polar summer or winter), the site returns no time rather than inventing one.
Source: Meeus, Astronomical Algorithms (2nd ed.), ch. 13 and 25; NOAA Solar Calculator equations.
Equinoxes and solstices
Computed astronomically using Meeus chapter 27 — the mean equinox/solstice expression plus the 24 periodic correction terms — rather than copied from a table that expires. Nowruz is derived from the March equinox as it falls in Iran (UTC+3:30).
Source: Meeus, Astronomical Algorithms (2nd ed.), ch. 27.
World calendars
The nine calendars — Gregorian, Islamic (Umm al-Qura), Hebrew, Persian (Solar Hijri), Chinese, Indian (Saka), Buddhist, Japanese and Ethiopian — are converted using the browser's ICU calendar implementations (en-u-ca-…). The Hijri converter additionally scans for an exact match rather than approximating from a day count, so a Hijri date maps to the precise Gregorian day in the Umm al-Qura tables.
Source: Unicode CLDR / ICU calendar data, via ECMA-402 Intl.
Easter and the Christian calendar
Western Easter uses the Anonymous Gregorian computus; Orthodox Easter uses the Julian computation with the 13-day offset that applies for 1900–2099. Good Friday, Ash Wednesday, Palm Sunday and Pentecost are derived from Easter by fixed offsets.
Source: standard computus algorithms as given in Meeus, ch. 8.
Weekdays, week numbers and leap years
Weekdays use the proleptic Gregorian calendar and are offered from 1583 onward — after the Gregorian reform — because before that the calendar in local use varied by country for well over a century. Week numbers follow ISO 8601: weeks start Monday, and week 1 is the week containing the year's first Thursday, which is why 1 January can legitimately fall in week 52 or 53 of the previous year. Leap years follow the full Gregorian rule, including the century exception.
Source: ISO 8601-1:2019.
Date differences and ages
Ages and year/month/day gaps are counted the way people count birthdays: whole months are added to the earlier date, clamped to each month's real length, and the remainder is the day count. This avoids the common bug where a start date late in a month produces a negative day figure.
One convention worth stating: a 29 February date clamps to 28 February in non-leap years, so someone born on a leap day is counted a year older on 28 February. Other conventions use 1 March. Neither is wrong — this site picks the clamped reading and says so rather than leaving you to guess.
What is not computed — stated plainly
Some lunisolar festivals depend on regional moon-sighting and cannot be derived by formula alone. Diwali, Holi, Chinese New Year, Vesak, Vaisakhi, Mahavir Jayanti and Ridván therefore come from verified published tables, not calculation. Chinese New Year is verified through 2050; Diwali and Holi through 2040; the others over shorter horizons. Beyond those dates the site would rather show nothing than show a guess.
Islamic dates follow the Umm al-Qura calendar and may differ by a day from local moon sighting. Jewish and Bahá'í observances begin at sundown the evening before. Hindu and Buddhist dates vary by region, and observance can differ by a day between cities. For congregational prayer, follow your local mosque's timetable — authorities round differently and may use slightly different twilight angles.
Where the calculations happen
Almost everything runs on your own device. The dates and times you type are not sent anywhere, and no calculation depends on this site's server being reachable or up to date. That is also why these tests can run in your browser at all: the same code that answers your questions is the code being tested.