The Handy Calendar

What day of the week is it? For calendar calculating like savant syndrome.

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Sum Day


Sum Day
Free (In-App Purchases)
iPhone / iPad
Removed from sale.

Price is subject to fluctuations.


A long time ago, in the age of paper, in order to efficiently utilize limited resources, there existed “perpetual calendars,” which arranged calendar information for days of the week in chart form to correspond with any month and date, making them usable in perpetuity.
While such calendars are for the most part unnecessary in the modern age, what if we could make such a thing into an app?

Of all times and places, this is likely the most simple of perpetual calendars. Information has been compactly organized into manageably memorizable sizes, allowing you to compute the days of the week with the simplicity of making case classification or addition calculations on paper. The thing we want to emphasize however, is that it’s not the app doing the calculations, but you, yourself. Using a tiny chart, and tiny calculations. Eventually, perhaps you will be able to “graduate” from this app, and become able to calculate the days of the week through mental arithmetic.

By nature, this is an app that’s short in practicality. But why not forget the burdens of this transient world once in a while, and have some fun with this calendar?

Recommended if you:

  • have an interest in calendars.
  • want to exercise your brain.
  • would like to surprise your friends with some special skills.

For more detailed usage instructions:

More detailed usage instructions for this app are available at Info(the button circling the “i”). Information on variables and symbols used in the app are included here as well, (although the word count is large) so please read beforehand.

About in-app purchases:

  • Tutorial

This will unlock the tutorial function. The tutorial will allow the user to input a date and learn how to use the app using that date.

Creator’s Comment

About 6 months ago, when I was completely bored, I was interviewed for a possible appearance on a television program, where I had the opportunity to talk about the finger counting calendar system(*1). As there was the chance that I might get to be on television, it was a very good wake up call for me. However, in the end, because it was thought to be “too academic,” my appearance was canceled. Through my many years of experience, I knew that there was meaning in being rejected, and so afterwards, I continued my work attentively, and about a month later I arrived at the “Transformation of Zeller formula,” of which one form has resulted in the work here. Actually, the formula derived from the transformation can be calculated using your fingers, but for this work I have sealed away the use of “fingers,” deciding to make it into a method of computing with the simplicity of making case classification or addition calculations on paper.

*1: Referring to the method of calculating days of the week using your fingers, introduced on this website. A special skill developed and acquired by the author.


Q: Do I need any preliminary knowledge?
Not particularly. However, as there will be descriptions regarding variables and divisibility, a first year middle school level of knowledge will be required.
Q: Is this calculation method really accurate?
There is a simple way to verify it. If you are interested, please refer to the information below.
Transformation of Zeller formula (PDF, 244KB)

Updated: 2015-11-04




If you have question or request about this App, please contact me from the following page.