Perpetual Calendar

📖 Tool Introduction

A perpetual calendar that puts the Gregorian date and the Chinese lunar date side by side, and stays useful for a very long stretch of years. It opens on the current month with today highlighted, arrows on either side step through months, and the year-month header takes you straight to a specific month when you need to jump.

Pick any day and the panel beside the grid opens up that date in detail: the lunar date, the four sexagenary pillars for year, month, day and hour, the solar term in effect, traditional and public holidays, and the day's auspicious and inauspicious activities from the old almanac. Below that sits an "On this day" section listing notable events that happened on that calendar date.

Every calculation runs inside your browser. Nothing is uploaded, there is no account to sign in to, and the calendar keeps working with the network off, on an intranet, or in airplane mode.

It turns out to be useful in more situations than you would expect: older relatives celebrate birthdays on the lunar calendar, so you need to convert a Gregorian birthday to its lunar equivalent; a family wants to pick a date for a wedding and would like to know what the day favours; you are writing something and need to confirm which date the Start of Spring fell on in a given year; or you simply want to know what day of the week you were born on, which animal signs applies and what zodiac sign that makes you.

The pillars row deserves an extra note. The year, month, day and hour pillars are derived through the traditional method: the first three come straight from the date, while the hour pillar is calculated from that day's heavenly stem. If all you want is your Chinese zodiac animal, the animal field alone answers it and you can skip studying what the four pillars mean.

A few details on the interface are easy to overlook. The small text in each calendar cell is the lunar day rather than the Gregorian one, solar term days carry their own marker, and hovering or clicking a date switches the panel on the right to match. Stepping into another year recomputes the pillars and almanac entries against that year's stem and branch, so what you see always belongs to the date you selected rather than being carried over from today.

✨ Key Features

• Gregorian and lunar dates shown together in every cell, so you never convert between the two in your head
• All four sexagenary pillars, for year, month, day and hour, computed and displayed at once with no separate tool needed
• The twenty-four solar terms marked on their exact dates, with the current term highlighted on the day it falls
• Traditional festivals, public holidays and solar term days all placed directly on the calendar grid
• Daily almanac guidance listing which activities suit the day, handy for anyone who checks dates before planning something
• An "On this day" feed of selected historical events attached to each date, readable without opening an encyclopedia

🚀 How to Use

1. Open the tool and it lands on the current month with today highlighted
2. Use the left and right arrows for adjacent months, or click the year-month header to jump to a specific one
3. Click any date to read its lunar date, pillars, solar term, holidays and almanac guidance
4. Scroll down for the historical events listed under "On this day"
5. Press the today button at any time to come back to the current date

⚠️ Things to Note

• Year range is limited: Lunar and solar term data covers 1900 through 2100. Dates outside that window are not guaranteed to be correct
• Almanac guidance is entertainment: The auspicious and inauspicious lists come from folk tradition, carry no scientific basis, and should not drive real decisions
• Solar terms carry small error: Solar term moments are computed for Beijing time and may differ from the astronomical moment by a few minutes
• History is a curated selection: The "On this day" entries are picked highlights rather than a complete chronology, so do not cite them as a historical source
• Everything runs locally: All calculations happen in the browser, the tool needs no network, and the dates you browse are never sent anywhere