Fixed Calendar Date Converter

🇯🇵 日本語 ｜ 🇬🇧 English

This tool lines up the dates of an event, year by year, in a fixed calendar (7 days × 52 weeks = 364 days) and in a calendar with leap years, so you can compare them. Year 1 uses the date you enter, and each later year moves forward by 364 days.

How to Use

Fields with a dashed border are still empty, and the number inside is the default. The tool calculates with these defaults if you enter nothing, so you can use them as they are. Only numbers can be entered. Click a field to select it, then turn the mouse wheel to change the number by 1 (the up and down arrow keys work too). If you turn the wheel on an empty field, the number changes from its default. The empty “Event end date” starts with the same numbers as the start date.

1. In “Start year,” enter the first year the event occurs. In this year, the month and day entered below become year 1.
2. In “Event start date,” enter a month and a day. For an event with a period, also enter a month and a day in “Event end date.” For a one-day event, leave the end date blank.
3. If needed, change the years before (back) and after (forward) the start year in “Years to show.” Forward years include the start year.
4. If the game changes the date at a time other than 00:00, choose it in “Date change time.”
5. Compare the fixed calendar and the calendar with leap years in each year’s card.
6. If you don’t know the start year, enter the year you tried (the base year) in “Reverse lookup.” Use “Candidates (each side)” to change how many candidates appear before and after the base.

How to Read the Screen

• Year cards: The heading shows the year and the event’s month and day. On a phone, the fixed calendar comes first and the leap-year calendar second, from top to bottom. On a wide screen, they sit side by side from left to right.
• In the fixed calendar, “364 days after year 1 (52 weeks)” is the number of days counted from the date in year 1. A line such as “2 days earlier than the same actual date” is the running total of the gap from the same date in the leap-year calendar, counted from year 1. The gap grows by 1 day each year, and by 2 days in a year that spans a leap day. For years before the start year, it reads “later.”
• Blue-bordered tag and ☆: These appear when the span from the event’s month and day to the same month and day of the next year is 366 days (a year that spans Feb 29). Other years get a “365 days” tag. They do not mean that the year itself is a leap year.
• Red box: This reads “Nonexistent date” when a date does not exist in that year of the leap-year calendar (for example, Feb 29 in a year that is not a leap year).
• Reverse lookup results: The base period appears first, followed by candidates made by subtracting and adding the period length. Under each candidate, you can see the start years whose event falls in that period. If a start year comes after the base year, it appears separately as “Start years after the base year.”

Differences Between the Two Calendars

• Fixed calendar (7 days × 52 weeks): A year is fixed at 364 days. The event date in year 1 is the month and day you entered for the start year. Each later year moves forward by 364 days, so the gap from the same actual date grows toward earlier dates by 1 day each year (by 2 days in a year that spans a leap day). Days of the week stay the same every year.
• Leap years: This is the actual calendar. When the span from the event’s month and day to the same month and day of the next year is 366 days (a year that spans Feb 29), a ☆ appears next to the year.
• Back: You can also show years before the start year. In the fixed calendar, the tool counts backward 364 days at a time.
• Feb 29 can be entered only when that date exists in the relevant year, which must be a leap year. For a start date, this is the start year. For an end date, it is the year of the end date (the year after the start year for a period that crosses the new year). For reverse lookup, the base year must also be a leap year.

Frequently Asked Questions

Q. What Happens if the End Date Is Earlier Than the Start Date?

The tool treats it as a period that crosses the new year. For example, if the start is Dec 20 and the end is Jan 5, the period runs until Jan 5 of the next year.

Q. What Range of Years Can Be Shown?

1900–2200. Back and forward years can total up to 100 years, and forward must be at least 1 year.

Q. How Do I Change the Numbers in the Fields?

You can type numbers (full-width digits are converted to half-width), or click a field to select it and turn the mouse wheel. Turning up increases the number, and turning down decreases it. A mouse changes the number by 1 per notch, and a touchpad changes it by 1 each time enough movement builds up. When no field is selected, the wheel scrolls the page. On devices without a wheel, such as phones, type the numbers.

Q. I Can’t Enter Feb 29.

Feb 29 can be entered only when that date exists in the relevant year. If the start date is Feb 29, make the start year a leap year (for example, 2028). If the end date is Feb 29 and the period crosses the new year (for example, Dec 20–Feb 29), use a start year whose next year is a leap year (for example, 2027). Reverse lookup also needs Feb 29 in the base year, so make the base year a leap year too.

Q. It Says “Nonexistent Date.”

This appears when a date does not exist in that year of the leap-year calendar (for example, Feb 29 in a year that is not a leap year). The fixed calendar moves forward 364 days at a time, so a date is still shown there.

Q. How Does the Display Change When I Choose a Date Change Time?

Times are added to the periods in the fixed calendar and the reverse lookup. For example, if you choose 06:00, the start shows “06:00” and the end shows “29:59” (which means 05:59 the next day). No times are added in the leap-year calendar. Some games treat a device time earlier than this time as the previous day.

Q. What Can the Reverse Lookup Tell Me?

Use it when you set the device date to a year (the base year) and the event did not occur. The tool adds and subtracts the period length (the number of days of the event) to and from the base period, and lists the next candidates that do not overlap. Each candidate shows the start years whose event falls in that period. Start years whose event falls in the base period are excluded, because the event did not occur.

Q. The Reverse Lookup Says “No Matching Start Year.”

This appears when no start year has its event in that period, because of leap years or other factors. The tool searches only 80 years before and after the base year (within 1900–2200), so candidates far from the base can show this message.

Q. Is My Input Saved?

Your input is not sent anywhere. It is not saved either, so the fields return to their defaults (start year 2027, event on Mar 15, and so on) when you reopen the page.

Q. The Dates Don’t Match the Actual Game.

This tool’s formula is one example of a method that matches dates by counting days. Change the date in the game to check how it actually behaves.

Notes

• Data as of: none (results are calculated on the spot from your input).
• Results are estimates. Before you rely on them, compare them with the dates in the actual game.
• Reverse lookup candidates are shifted based on the number of days in the start year. For an event whose period spans Feb 29, the number of days differs by 1 from year to year, so a candidate can be off by 1 day from the actual number of days in the base year. Games count days in different ways, so treat the candidates as estimates.
• Your input is not sent anywhere.