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 == {| class="wikitable" |+ 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 |- !colspan=32| 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 || {{n/a}} |- ! 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 || {{n/a}} |- ! 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 || {{n/a}} |- ! 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 || {{n/a}} |- ! 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 |- !colspan=32| 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 || {{n/a}} |- ! 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 || {{n/a}} |- ! 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 || {{n/a}} |- ! 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 || {{n/a}} |- ! 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 |}

{| class="wikitable" |+ Quarter layout by weeks ! Q1 month !! Q2 month !! Q3 month !! Q4 month ! Mon !! Tue !! Wed !! Thu !! Fri !! Sat !! Sun |- !rowspan=5| January !!rowspan=5| April !!rowspan=5| July !!rowspan=5| October |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||colspan=5 {{n/a}} |- !rowspan=5| February !!rowspan=5| May !!rowspan=5| August !!rowspan=5| November |colspan=2 {{n/a}}||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||colspan=3 {{n/a}} |- !rowspan=5| March !!rowspan=5| June !!rowspan=5| September !!rowspan=5| December |colspan=4 {{n/a}}||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 |}

== 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 == <!-- TODO: link back to this article --> * List of calendars: Reform calendars * Calendar reform: Specific proposals * Armelin's calendar

== Sources == <references/> <!--- After listing your sources please cite them using inline citations and place them after the information they cite. Please see http://en.wikipedia.org/wiki/Wikipedia:REFB for instructions on how to add citations. ---> * [http://sundaymagazine.org/2010/06/a-proposed-plan-for-an-invariable-calendar/ A Proposed Plan For An Invariable Calendar], Sunday Magazine, David Friedman, June 25, 2010, at 9:02 am, [http://www.sundaymagazine.org/wp-content/uploads/19100626-2-a.pdf PDF]{{Dead link|date=November 2025 |bot=InternetArchiveBot }} * [https://query.nytimes.com/gst/abstract.html?res=F60916FC395417738DDDAF0A94DE405B808DF1D3 A PROPOSED PLAN FOR AN INVARIABLE CALENDAR], June 26, 1910, The New York Times, Hedley P. Somner, [https://timesmachine.nytimes.com/timesmachine/1910/06/26/104942065.pdf PDF] * [http://adsabs.harvard.edu/full/1912PA.....20..232W The Reform of the Calendar], Popular Astronomy, vol. 20, pp.&nbsp;232–236, 04/1912, Ralph E. Wilson, [http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1912PA.....20..232W&defaultprint=YES&filetype=.pdf PDF] ** [http://adsabs.harvard.edu/full/1912PASP...24..113W The Reform of the Calendar], Publications of the Astronomical Society of the Pacific, Vol. 24, No. 141, p.&nbsp;113, [http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1912PASP...24..113W&defaultprint=YES&filetype=.pdf PDF] * [https://books.google.com/books?id=ficDAAAAMBAJ&dq=Grosclaude+calendar&pg=PA33 Wanted—A Brand-New Calendar], Popular Science, Jan 1927, pp.&nbsp;33–34, 134–135, Frank Parker Stockbridge * [https://archive.org/stream/reformcalendar00philgoog/reformcalendar00philgoog_djvu.txt THE REFORM OF THE CALENDAR], ALEXANDER PHILIP, 1914, [https://books.google.com/books?id=KOusvFSUkbkC&dq=Grosclaude+%22Journal+of+Horology%22&pg=PA57 Diagram], [https://books.google.com/books/download/The_reform_of_the_calendar.pdf?id=KOusvFSUkbkC&output=pdf&sig=ACfU3U1U3dAGai_9zKJ6mRhMQdpTFRfYXg PDF]

Category:Proposed calendars