Month 12, day 2—then New Year’s Day. Where did the missing dates go?
In Meiji-era Japan, the day after the second of the twelfth month became January 1. Place the old and new calendars on the same timeline, and you can see what jumped ahead and what continued uninterrupted.
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The second day of the twelfth month of Meiji 5 in the old calendar and January 1 of Meiji 6 in the new calendar were consecutive days. Expressed in the Gregorian calendar, they are December 31, 1872, and January 1, 1873. What changed abruptly was the way dates were assigned, not the passage of time.
New Year’s Day arrived overnight?
Go to bed on the second day of the twelfth month and wake up on New Year’s Day. That is the kind of date change that occurred when Japan reformed its calendar in the Meiji era. But the story becomes confusing if we assume that the old calendar’s twelfth month and Gregorian December are the same month. The final day under the old calendar was the second day of the twelfth month of Meiji 5. The following day was January 1 of Meiji 6 under the new calendar. The National Diet Library identifies that old-calendar date as December 31, 1872, in the Gregorian calendar. Write both days in the calendar used today, and December 31 is followed by January 1. Several weeks of time did not vanish in between. The rules for assigning a year, month, and day to those same passing days changed.
A calendar boundary
Move one day along the timeline: how do the old and new calendar labels change?
Two consecutive days on a timeline. Compare the “calendar labels” attached to the same day above and below.
The first dayMove forward just 1 day →The following day
Before the change
Old calendar then in use: Meiji 5
Month12Day2
Through this dayOld-calendar dates were used
The following day
Newly adopted calendar: Meiji 6
Month1Day1
Old calendar, for comparisonMeiji 5, month 12, day 3This labels the same day in the old calendar for comparison. It is not the official date used after the new calendar was adopted.
Old calendar then in use: Meiji 5, month 12, day 2 = Gregorian calendar: December 31, 1872
The date labels jumped ahead. The two days remained just 1 day apart.
Only the two days at the documented boundary are shown. This is not a converter for arbitrary old-calendar dates.
Read the same two days on two scales
Set the two days on either side of the change next to each other. The first is “the second day of the twelfth month of Meiji 5 under the old calendar then in use / December 31, 1872, in the Gregorian calendar.” The second is “January 1 of Meiji 6 under the newly adopted calendar / January 1, 1873, in the Gregorian calendar.” If we express the second day in the previous calendar, it corresponds to the third day of the twelfth month of Meiji 5. This third label is a conversion for comparison. It does not mean that Japan’s official calendar continued to use the third day of the twelfth month after the reform. Move your finger just one day to the right and read the dates attached above and below it. Think of different labels attached to the same point in time, and the large jump in the written numbers becomes less misleading.
The old calendar did not follow only the Moon
The Tenpo calendar used until then was a lunisolar calendar. It divided months according to the Moon’s phases while also trying to keep them aligned with the cycle of the seasons. Twelve lunar cycles amount to about 354 days, roughly 11 days shorter than the solar year over which the seasons return. Let that difference accumulate unchecked, and numbered months drift away from their seasons. A lunisolar calendar therefore has a way to adjust by occasionally inserting a leap month. Japan’s present calendar is a solar calendar. The starting point is to understand the reform as a move from a calendar linking both the Moon and the seasons to one based on the solar year. The idea that people in the past simply miscounted the days and corrected the total all at once cannot explain this distinction.
Next year’s calendars were already in circulation
According to a calendar exhibition at the National Astronomical Observatory of Japan, the reform was announced on the ninth day of the eleventh month of Meiji 5 in the old calendar. By then, calendars for the following year based on the Tenpo calendar had already begun to circulate. Here, too, the date “month 11, day 9” must not be read as November 9 in the modern Gregorian calendar. New rules had caught up with next year’s calendars, which had been prepared under the old rules. If you had both versions in front of you, copying appointments by matching only the month and day would be risky. You would need to ask not only “What is the date?” but also “Which calendar is this date written in?” A change of calendar altered more than a calculation table: it also changed how people communicated dates.
A diary records someone crossing the boundary
The National Diet Library presents a diary by Shishido Tamaki that marks the last day of the old calendar as a boundary. The next day’s entry also mentions using the solar calendar for the first time and calling the date January 1. Put a timeline of institutional change beside one person’s daily record, and “calendar reform” becomes a much more tangible event. Yet this one diary does not show that everyone in Japan received the news on the same day or made the transition with the same feelings. What it does establish is that this writer consciously recorded the boundary. A major institutional change and the ways people received it in their own lives need to be examined through distinct sets of evidence.
What changed was a shared rule for dates
Why make the change? Alongside the operation of the calendar itself, the National Astronomical Observatory of Japan discusses the diplomatic convenience of aligning dates with other countries and the significance of signaling a new era. It is more careful to understand the reform as an institutional change shaped by several overlapping circumstances than to explain everything through a single benefit. A shared calendar lets people far apart compare dates on the same basis. The next time you read a date in an old letter or diary, first check whether it uses the old or new calendar, then convert dates to the same calendar before comparing them. Beneath the surprise of “the second day of the twelfth month, then January 1” lies a question about how a society counts and shares the continuous passage of time.
Sources and verification notes
This article is an original synthesis of primary sources and explanations from specialist institutions.
Generative AI assists with research, structure and English translation. Claims are compared with sources and editorial checks are performed; independent specialist and native-English reviews have not yet been completed.
- National Diet LibraryShishido Tamaki: from the diary entry for the second day of the twelfth month of Meiji 5 (December 31, 1872) ↗ (opens in a new tab)
- Calendar Computation Office, National Astronomical Observatory of JapanCalendar Wiki: lunisolar calendars ↗ (opens in a new tab)
- Calendar Computation Office, National Astronomical Observatory of JapanPermanent Exhibition No. 30: calendar distribution in the year of reform ↗ (opens in a new tab)
- National Astronomical Observatory of Japan150 years since the calendar reform, part 2: changing times and Japanese calendars, from the early modern to the modern and contemporary periods ↗ (opens in a new tab)
Sources checked:
The date boundary was checked against the National Diet Library’s diary presentation and explanations from the National Astronomical Observatory of Japan. The second and third days of the twelfth month and the ninth day of the eleventh month of Meiji 5 are old-calendar dates; they are not treated as Gregorian date strings. The diary was consulted through the transcription and commentary published by the National Diet Library, not through an independent reading of the original handwriting. This article does not verify the relative importance of the motives for reform, its effects on salaries or public finances, or the total number of calendar dates omitted.
Rights, status, and revision history
- Status
- Free public edition. Preparation and revision history is retained below.
- 0.1.1
- Rights to the text and diagrams
- The original Japanese text is an independently written summary. The SVG illustrations and conceptual diagrams were newly created. No external images, original source text, or audio recordings are reproduced.
- How sources are used
- Sources are referenced by links only. All rights to each source remain with its respective rights holders.
- · v0.1.0 — Checked primary sources and explanations from specialist institutions, and prepared a private preview article with a guiding question, comparisons, conditions, and references. Added an original explanatory exhibit. Publication has not been approved.
- · v0.1.1 — Added links explaining how to explore the new questions in v15. The existing article text, comparison conditions, sources, and source verification dates are unchanged.
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