Improved Perpetual Calendar (to the Year 2124)




Introduction: Improved Perpetual Calendar (to the Year 2124)

Most (non-computer) perpetual calendars do not render good representations of a month.For example, see and Some show the month with a variable starting day of the week. Others show the month with the maximum number of days instead of the actual number of days in the month. In general, they usually require a complicated table lookup to determine how to set the calendar view.

This calendar attempts to address this by:

1) Eliminating table lookups by providing viewports that show only relevant information.

2) Providing an accurate representation of a month, always starting with Sunday and showing only the days that actually exist in the month.

3) Providing a logical process for setting the calendar: set the year, set the month, set the starting day of the month, then mask off the days that don't appear in that month.

The calendar is double sided to reduce the diameter of the overall calendar.A single sided version could be created by putting the year and month information on outer rings. That was my initial design, but it got too big, and I was hoping to create a smaller, pocketable version like some of the older ones. See

This calendar would be easy to convert to laser cut and laser etched metal sheets, which would yield a keepsake that would work for more than a century. Replacing the year disk would make it work for another century.

Step 1: Print PDFs

 Print out the PDFs onto individual card stock sheets.

Step 2: Cut Out

Cut out the sections designated in red lines. These are small and large irregular sections.
Cut away the outer red ring from each disk.

Step 3: Punch Center Holes

Punch a small hole in the center of each circle. I’d start with a pin first to get good alignment of the disks.
Carefully align Disk 2 Back and Disk 2 Front by their center holes and then glue them together, using the light blue marks for alignment.

Step 4: Assemble Back and Front Masks

Carefully align the Back Mask and the Front Mask by their center holes (temporarily insert a pin) and then glue or staple them together only at the very bottom.

Step 5: Assemble Year Disk

Insert the Year Disk (Disk 1)

Step 6: Disk 2 - Month Disk

Insert the Month Disk (Disk 2) behind Disk 1 with the Back disk facing to the Back Mask.

Step 7: Flip Over

 Flip the assembly over. Note Disk 2 (the Month disk) with the Front Disk facing the front. February is visible in the port.

Step 8: Insert Disk 3 - Calendar

Insert Disk 3, the Calendar disk.

Step 9: Insert Disk 4

Insert Disk 4 - the days in month disk. Note that F and Triangle are in the view ports.
Put a pin, paper clip or a small plastic bolt through the center of all the disks and covers.


1) Set the year on the back of the calendar.
2) Set the month and note the letter and symbol that is visible.
3) Turn over the calendar and set the letter.
4) Set the symbol.

Step 10: Explanation


1) Back Mask
This masks off all other years and months so that only the relevant year and month is visible. It does not move and the visible information is shown at the top of the mask.

2) Year Disk (Disk 1)
This has 14 segments to it. This is because a year can start on any 7 of the days of the week and a year can be either a leap year or not (7 * 2 = 14). The year is grouped with all the other years in the future that have the same configuration.  This disk provides a viewport to only the month information for that year configuration. This is a moveable disk that basically only changes each year.

3) Month Disk (Disk 2)
This is a double-sided disk with 12 segments on each side for each month. On the back, the month is displayed centered at the top in each of the 14 possible year configurations. Each year configuration unmasks just the month data for that year. Thus, each month has calendar configuration information for each of the 14 possible year configurations.

On the front of this disk is the month name to simply be displayed under the calendar.

4) Calendar Disk (Disk 3)
This disk works with the next two layers to display a correct calendar for the month. It is designed to provide all the information needed for any possible calendar configuration. This disk has a "handle" on it, so it does not freely spin all the way around.

5) Days in Month Mask (Disk 4)
This disk, when positioned to view the desired symbol, will mask off days from the end of the month so that the month will have 31, 30, 29 or 28 days. 31 days corresponds to the circle symbol, 30 days is the square, 29 days is the star and 28 days is the triangle. Once the mask symbol is selected, this disk moves with the calendar disk. This disk has a "handle" too, and does not freely spin all the way around. The handles of Disk 4 and Disk 3 coincide for a 31 day month because it is the most common.

6) Front Mask
Provides a mask for the calendar view so that only the 7 days of the week are shown in the proper configuration.  With the correct letter in the viewport, the correct month view is shown. Also, a viewport is provided to show the selected month’s name. This mask does not move and is attached to the Back Mask.


This is still a prototype. I am seeking input on the design. This is why the grids are still left on the printouts. PDFs without the grids could be made available.

Look at the spreadsheet to see the data used to create these disks. Years follow a 28 year repeating cycle (except for century years).

The PDFs are printed with polar grids to show how they align. The polar grids were invaluable templates in developing this calendar. The custom grids were created on the site,

2 People Made This Project!


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1 year ago

On the back in the year window, you have 2 years showing, can the second be blocked out so only the year you want is shown?
Thanks for this, I've been looking everywhere for one and this one is the best.


1 year ago

Years after 2100 are shown in the wrong sectors. Yes, 2100 is not a leap year, and so it belongs in the 2094 sector. However, the years 2101 to 2124 are shown as if 2100 were to be a leap year.

2101, 2107, and 2118 belong in the 2095 sector, not the 2090 sector.
2102, 2113, and 2119 belong in the 2090 sector, not the 2091 sector.
2103 and 2114 belong in the 2091 sector, not the 2097 sector.
2104 belongs in the 2092 sector, not the 2076 sector.
2105, 2111, and 2122 belong in the 2099 sector, not the 2094 sector.
2106, 2117, and 2123 belong in the 2094 sector (with 2100), not the 2095 sector.
2108 belongs in the 2096 sector, not the 2080 sector.
2109 and 2115 belong in the 2097 sector, not the 2098 sector.
2110 and 2121 belong in the 2098 sector, not the 2099 sector.
2112 belongs in the 2072 sector, not the 2084 sector.
2116 belongs in the 2076 sector, not the 2088 sector.
2120 belongs in the 2080 sector, not the 2092 sector.
Finally, 2124 belongs in the 2084 sector, not the 2096 sector.


Question 2 years ago

Hello, this calendar looks amazing! I have however two questions. Is it possible to add more years, like the whole 19th, 20th, 21st century? Also, you mention that originally you designed it so that all information could be put on one side – can you explain, how to do it? Thank you!


Answer 2 years ago

As long as you can read the years, the calendar's font can be shrunk. However, putting more than 6 or 7 year numbers in the window makes it harder to find them. The most number I would recommend is about 100.


5 years ago

I've made one using a descendent of your design. My version removed the outer masks and made the viewports output only.

I also have tutorials make perpetual calendar (used until 2050), the simplicity of this friend.


7 years ago on Step 9

OMG. This is the best calendar I've found in the internet. thank you so much.


9 years ago on Introduction

Great calendar! I am developing a series of activities/models for Mathematics of Planet Earth ( is the official website and my own is I would like to provide your pdf files on my website so that my readers can download them easily. I will obviously give full credit to you and the model will be available freely for all users. I trust that would be OK.


Reply 9 years ago on Introduction

Where do you want me to send the files? I will send you PDFs with the polar grids removed.


Reply 9 years ago on Introduction

Can you email them to me? I am planning to package them into a single pdf along with the instructions so that my users can download them easily.

After seeing your instructable, I designed a couple of perpetual calendars, which i will upload in a few days time.

Thanks! Sitaram


Reply 9 years ago on Introduction

Sure, I could provide the PDFs. They are attached to the Instructible. I could include the Excel file that I used to create the data represented in the calendar. That was probably the most enlightening part of the project.


9 years ago on Introduction

Interesting design, you could produce it in serie, and market it.