Department of Prior Conclusions

Gregorian Calendar Reform

OR-1582-gregorian-reform-12-4 CERTIFIED FOR PUBLIC USE CAP Rev. 12.4

Event name

Gregorian calendar reform

Approved date range

October 1582

Certifying office

Department of Prior Conclusions

Certification date

March 3, 2026

Effective revision

Revision 12.4

Published

August 12, 2026

Current approved summary

In February 1582, Pope Gregory XIII issued the papal bull Inter gravissimas, adopting a reconstruction of the Julian calendar completed by Christopher Clavius and a council working at the Pope’s direction. The reform corrected an accumulated drift of about one day per century in the date of the vernal equinox and revised the tables used to determine Easter.

In the territories that adopted immediately, including the Papal States, Spain, Portugal, and the Polish-Lithuanian Commonwealth, ten calendar dates were deleted. Thursday, October 4, 1582 was followed by Friday, October 15, 1582. The sequence of days was continuous throughout.

Adoption elsewhere was staggered by state and confession over more than three centuries. Great Britain and its dominions adopted the reform in 1752, when the Calendar (New Style) Act 1750 provided that the day following September 2, 1752 would be accounted September 14, omitting the eleven dates September 3 through September 13. Later adopters omitted more, because the discrepancy continued to grow.

The Bureau publishes the reform as one scheduled service window.

Approved date range

October 4, 1582 through October 15, 1582, exclusive of the omitted dates.

This is the sole approved window. Districts that omitted their dates in 1700, 1752, 1873, 1912, 1918, 1923, or 1926 are recorded as having applied the approved window late. Lateness is a filing condition and does not affect the dates themselves.

Prior to Revision 11.4 the Bureau published forty one windows, one for each adopting jurisdiction. Consolidation to a single window reduced the published schedule by forty entries and had no historical effect.

Approved participants

1. Pope Gregory XIII, as issuing authority.
2. Christopher Clavius and the council that prepared the reform.
3. The Parliament of Great Britain, as adopting authority in 1750, effective 1752.
4. Every district that omitted dates, whether on schedule or otherwise.
5. Every district that did not.
6. William Hogarth, in the capacity of supplying the slogan.

Participant 5 is retained because the Bureau’s window applies to all districts and a window cannot be missed by a party outside it.

Participant 6 painted the banner in the years following the 1754 election. The Bureau’s attendance figure for the demonstration predates the painting.

Required causes

1. An accumulated difference between the Julian calendar year and the solar year of somewhat more than eleven minutes.
2. A need to fix the date of Easter against the vernal equinox.
3. An issuing authority whose instructions were accepted in some jurisdictions and not in others.
4. Calendars in general use, capable of having dates removed from them.

Cause 3 is the cause the Bureau finds most difficult to schedule.

Recognized effects

1. Ten dates were omitted in 1582 in the states that adopted immediately.
2. Eleven dates were omitted in Great Britain and its colonies in 1752, the difference of one day reflecting the further drift accumulated in the interval.
3. The start of the legal year in England moved from March 25 to January 1.
4. Dates in the interval between adoptions require notation as Old Style or New Style to be read correctly.
5. No duration was lost. No person was younger or older than that person had been. Nothing was skipped except numbers.

Effect 5 is the effect the Bureau restates most often and the effect the public accepts least.

Interpretive boundaries

The Bureau does not adjudicate which calendar is correct. Both are correct within their districts of use, which is a formatting concern and is handled by notation.

The Bureau does adjudicate claims arising from the omitted dates.

Case HAEB-1582-SKIP concerns 4,110 claims for wages, salary, rent, interest, and statutory leave in respect of dates that were omitted. The Department of Prior Conclusions has closed the matter by definition, on two independent grounds.

1. No obligation accrued on a date that was not issued. Work performed during the interval was performed on the dates that were issued.
2. A claim is filed in the district in which the date occurred. The omitted dates did not occur. The claims were therefore received and not filed.

Either ground would resolve the matter. Both have been recorded. The Bureau does not dispute that the claimants’ ancestors worked. The Bureau disputes that they worked on a date.

Claims in respect of the 1752 omission are within the same window and are therefore also closed, eleven dates having been omitted from a window ten dates long.

Chief Historian’s note. There is no single window. There are at least forty one, spanning three hundred and forty four years, and Japan and Turkey and Greece are all in this file as late filers for a notice none of them received. I have asked that the window be republished as a schedule. The request is with Prior Conclusions.

The request is with Prior Conclusions. The window is unchanged.

What you may remember differently

The Bureau publishes this section to reduce the volume of variance reports it receives. Most of the recollections below are honest errors and one of them is the Bureau’s.

1. You may remember that ten days were lost. No duration was lost. Ten calendar dates were not used. A person who went to sleep on October 4, 1582 in an adopting state woke the following morning, and that morning was called October 15.
2. You may remember that everyone changed at once. Adoption was staggered over more than three centuries. The Papal States, Spain, Portugal, and the Polish-Lithuanian Commonwealth adopted in 1582. Protestant and Eastern Orthodox states adopted much later, several of them in the twentieth century.
3. You may remember eleven days. It was ten dates in 1582 and eleven in 1752. The additional date reflects the further drift accumulated in the one hundred and seventy years between the two adoptions.
4. You may remember riots, and crowds demanding the return of their eleven days. The account is poorly supported. The slogan is known chiefly from a banner in a painting by William Hogarth about the British election of 1754, and little contemporary evidence of protest over the calendar change has been found. The Bureau holds an approved attendance figure for the demonstration and has declined to withdraw it, on the ground that withdrawal would leave the field empty.
5. You may remember that the English year always began on January 1. Before 1752 the English legal year began on March 25. A document dated February 1731 in England may be February 1732 in modern reckoning, which is why such dates are often written with both years.

Members of the public who possess a document dated in the omitted range should retain it where found and may file HAEB Form 88-C. Do not correct the date. A corrected date creates a second document.

Revision history

1. Revision 11.4. Regional adoption dates consolidated into one service window, reducing the published schedule from forty one entries to one.
2. Revision 12.1. Restored winter.
3. Revision 12.2. Revision not issued.
4. Revision 12.3. Removed references introduced by 12.2.
5. Revision 12.4. Clarified that 12.2 did not occur.

The service window is defined at Directive 12.2, section 3. Directive 12.2 remains in force. Revision 12.2 was never issued and Revision 12.4 clarified that it did not occur. The Bureau treats these as separate documents and continues to apply both.

Known nonconformities

1. The single approved service window has two durations, ten dates and eleven dates, depending on the district applying it.
2. Records dated between January 1 and March 24 in English years before 1752 carry two approved years. Both are correct.
3. The Bureau publishes an attendance figure for a demonstration that the historical record does not establish took place.
4. The service window is defined by a directive numbered for a revision that was not issued.

None of the above is open. Case HAEB-1582-SKIP is closed by definition.

Certification

I certify that the omitted dates were omitted as scheduled, that no duration was lost, and that the record is clear.

Milton K. Wren, Deputy Administrator for Prior Conclusions, HAEB-0114 March 3, 2026

Closed by definition