Date Inspector

Almanac Plate

Famous Date Probes

Tap to inspect notable dates from world history.

Date → Almanac Readings

For just today, see Today's Date; for the day-of-the-week alone, see Day of Week.

The Meeus JDN Formula

Zodiac and Season Reference

No saved inspections yet. Pick a date and tap "Save to History" to remember up to eight almanac entries.

How to Read the Almanac Plate

1. Pick any date in the picker. The almanac defaults to today; type or click in a different YYYY-MM-DD to inspect a historical, present, or future date.
2. Read the Gregorian (top) and Julian (Old-Style) dates. The Julian row reflects the 13-day modern offset and is the equivalent used in pre-reform records.
3. Note the ISO 8601 week date and ordinal (day-of-year). These two readings are what SAP, Workday, DHL, and aviation systems consume.
4. Check the season and tropical zodiac panels. The element + ruler annotation follows Ptolemy's Tetrabiblos convention.
5. Save the inspection. The last eight dates persist in your browser's localStorage for cross-reference.

A Brief History of the Civil Date

In 2026, a historian transcribing a 17th-century Spanish baptismal register encounters a date written as 5 March 1684 (Julian). She needs the Gregorian equivalent, the Julian Day Number for cross-referencing British naval logs (which switched calendars in 1752), the day-of-the-week the baptism actually fell on, and the season so she can verify whether the candle-stub mentioned in the margin matches a winter or spring rite. Date Inspector resolves all four readings of any date on a single almanac plate.

The Gregorian calendar was promulgated by Pope Gregory XIII's 1582 papal bull Inter gravissimas, which dropped 10 days (Thursday 4 October 1582 was followed by Friday 15 October 1582 in Spain, Portugal, Italy, and the Polish-Lithuanian Commonwealth) and tightened the leap rule to exclude century-non-quadcentennial years. Britain and her colonies adopted the reform on 14 September 1752, jumping over 11 days (an additional day accumulated since 1582). Russia held the Julian calendar until 1918, and Greece until 1923. The 13-day Julian-Gregorian gap as of 2026 traces straight back to Caesar's 45 BCE reform.

The Julian Day Number (JDN) was proposed by Joseph Scaliger in De emendatione temporum (1583) and revised by John F. Herschel in 1849. It is a continuous count of days from noon UT on 1 January 4713 BCE in the proleptic Julian calendar. Astronomers, archaeologists, and chronologists use JDN because it has no leap-year exceptions and no calendar-reform discontinuities — 2 April 1684 Julian is JDN 2,331,953 regardless of which civil calendar your source uses. NASA's Horizons system, the Smithsonian Astrophysical Observatory catalogues, and the IAU all default to Julian Day for ephemeris work.

ISO 8601 was ratified by the International Organization for Standardization in 1988 and most recently revised in 2019. It defines the YYYY-MM-DD Gregorian date, the YYYY-Www-D week date (where week 01 contains the year's first Thursday), and the YYYY-DDD ordinal date. ISO weeks run Monday-to-Sunday, unlike US convention which runs Sunday-to-Saturday. Modern ERP systems (SAP, Workday), shipping carriers (DHL, Maersk), and academic calendars schedule by ISO week. Date Inspector returns all three ISO representations so a user can copy whichever the destination system expects.

Astronomical seasons are bounded by the equinoxes (vernal: c. 20 March, autumnal: c. 22 September) and solstices (summer: c. 21 June, winter: c. 21 December) in the Northern Hemisphere. The exact moment shifts by hours each year due to the 365.2422-day tropical year and is computed by ephemerides like VSOP87 (Bretagnon and Francou, 1988) and the more recent ELP/MPP02 (Chapront and Francou, 2003). Date Inspector uses the conventional civil-calendar bounding (Mar 20 / Jun 21 / Sep 23 / Dec 21) which is correct within a day for any year between 1900 and 2100.

Zodiac sun signs were codified by Ptolemy in the Tetrabiblos (c. 150 CE) and the date boundaries on this plate follow the tropical-zodiac convention used by most Western newspaper horoscopes: Aries 21 March to 19 April, Taurus 20 April to 20 May, and so on through Pisces 19 February to 20 March. Indian Vedic astrology uses the sidereal zodiac, which is currently offset by about 24 degrees from the tropical zodiac due to the precession of the equinoxes (discovered by Hipparchus c. 130 BCE). Date Inspector returns tropical-zodiac sign; researchers needing sidereal can subtract ~23 days from the boundary dates.

The seven-day week and its weekday names trace to the Babylonian-Hellenistic planetary week (Vettius Valens, 2nd c. CE) and were Romanised as dies Solis through dies Saturni. Constantine's 321 CE edict made Sunday the official Roman day of rest. The Christian liturgical week is Sunday-first; the Jewish week (Shabbat = Saturday rest) and the ISO civil week (Monday-first) differ. The weekday returned by JavaScript's getDay() is Sunday-first (0-6); Date Inspector surfaces both the English name and the Latin equivalent for historians transcribing pre-modern documents.