13 Month Fixed Calendar

2.9
153 reviews
Content rating
Everyone
10K+
Downloads
Content rating
Everyone
Learn more
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About this app

The International Fixed Calendar is a calendar system where every year aligns with the same days of the week and dates, ensuring consistency in weekly patterns. It was first proposed by Moses B. Cotsworth in 1902. This calendar divides the year into 13 equal-sized months, each consisting of 28 days, resulting in a total of 364 days. Leap years are calculated similarly to the Gregorian calendar, with an additional day added every four years. However, in the International Fixed Calendar, the extra day is inserted between June 28 and July 1, known as "Sol," effectively creating a 365-day leap year.

While the month names align with those of the Gregorian calendar, an extra month called "Sol" is inserted between June and July. This arrangement starts each year on a Sunday and ends on a Saturday, ensuring that the first day of each month falls on a Sunday and the last day on a Saturday. This consistency aids in planning and daily routines.

The International Fixed Calendar enhances calendar consistency by ensuring that the same dates align with the same weekdays each year.
Updated on
May 13, 2025

Data safety

Safety starts with understanding how developers collect and share your data. Data privacy and security practices may vary based on your use, region, and age. The developer provided this information and may update it over time.
This app may share these data types with third parties
App activity and Device or other IDs
This app may collect these data types
App activity, App info and performance, and Device or other IDs
Data is encrypted in transit
Data can’t be deleted

Ratings and reviews

2.8
148 reviews
andres beeco
August 18, 2025
I've been looking for an app like this for a while now. Though, if it's possible, I would love for the developer to add 3 features to it if it's possible. 1. Allow for users to change the names of the months. 2. Allow for users to mark dates on the calendars. 3. When a date is marked, the user can write on and save a pop up note explaining the occasion.
11 people found this review helpful
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Allison Doucet
May 1, 2026
This app is mostly accurate. The only thing that seems incorrect is the year day (day out of time for rest) being at the end of December. from all the research I've done on the old calendar year day is supposed to happen between March 28th and April 1st because April 1st used to be the New year before new years was changed to January 1st. (apparently why April first is called April fools) but since year day is a day out of time for rest the dates in the app are still correct.
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elliotonline
February 3, 2026
I thought this would be a normal calendar app (in that you would be able to put in reminders and events) but unfortunately it just shows the Universal Fixed Calendar, current date, and what day it is in the Gregorian. I was thinking perhaps I could use this calendar for personal scheduling of housework, meds, etc. but sadly there's no way for me to do so using this app.
3 people found this review helpful
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What’s new


## Version 2.2.3

This version includes significant improvements aimed at enhancing the app’s core functionality and user experience.

- User interface updated.
- Fixed issues related to Day of the Year and Leap Day.
- Settings screen added.
- Added Dark Mode and custom theme color options.
- Added info screen explaining the 13-month calendar and its history.
- Added support for German, French, Spanish, Portuguese, Polish, Italian, Russian, Chinese, and Arabic languages.