In April 1900, Professor L. A. Grosclaude of Geneva proposed the Invariable Calendar, New Era Calendar, or Normal Calendar with 12 months and four 91-day quarters of exactly 13 weeks. The final month of each quarter would have 31 days, the others 30 days each. Quarters all began on a Monday. An additional day, termed New Year's Day, that was not any day of the week and not part of any month, would occur between December 31 and January 1. Another such day is inserted between June 31 and July 1 on leap years. This placement of the leap day is virtually the only difference to Armelin's calendar which has it at the end of the year.

This model would be a perennial calendar, with each date occurring perennially on the same day of the week. Grosclaude lists many business advantages to this. Three monthly days important to businesses, the 1st, 15th, and 30th would always occur on the same 3 days of the week, respectively, and additionally would never occur on a Sunday, for example. It became the model for The World Calendar, promoted by Elisabeth Acheils and The World Calendar Association since 1930.

Structure + Year layout by months and days Month 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 New Years Day January Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu February We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th March Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su April Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu May We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th June Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Leap Day July Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu August We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th September Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su October Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu November We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th December Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Mo Tu We Th Fr Sa Su Month 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

+ Quarter layout by weeks Q1 month Q2 month Q3 month Q4 month Mon Tue Wed Thu Fri Sat Sun January April July October010203040506070809101112131415161718192021222324252627282930colspan=5 February May August Novembercolspan=2 010203040506070809101112131415161718192021222324252627282930colspan=3 March June September Decembercolspan=4 01020304050607080910111213141516171819202122232425262728293031

Criticism The Sabbatarian objection, that the strict cycle of the seven-day week is interrupted; sometimes there are seven days between sabbaths, instead of the usual six. Difficulties of defining the dates of New Year's and Leap Days when they don't belong to any month or week.

Other calendars and proposals

List of calendars: Reform calendars Calendar reform: Specific proposals Armelin's calendar

Sources

A Proposed Plan For An Invariable Calendar, Sunday Magazine, David Friedman, June 25, 2010, at 9:02 am, PDF A PROPOSED PLAN FOR AN INVARIABLE CALENDAR, June 26, 1910, The New York Times, Hedley P. Somner, PDF The Reform of the Calendar, Popular Astronomy, vol. 20, pp. 232–236, 04/1912, Ralph E. Wilson, PDF The Reform of the Calendar, Publications of the Astronomical Society of the Pacific, Vol. 24, No. 141, p. 113, PDF Wanted—A Brand-New Calendar, Popular Science, Jan 1927, pp. 33–34, 134–135, Frank Parker Stockbridge THE REFORM OF THE CALENDAR, ALEXANDER PHILIP, 1914, Diagram, PDF