Ignoring what is current status quo and how complicated would it be in reality to change calendar system, what is the best calendar system that makes the most sense for humanity to use from a neutral perspective?

Sonar Pro

Answer: I maintain my position that the World Calendar is the best calendar system for humanity from a neutral perspective, as none of the other models' arguments provided compelling reasons to change.

The other models' responses largely converged on similar perpetual solar calendars, with most advocating the International Fixed Calendar (IFC, 13x28 days) or 12-month variants like the World Calendar (equal quarters with blank days). Grok 4.20 and GPT-5.4 favored 12-month structures with 91-day quarters and blank days, closely aligning with my World Calendar endorsement, emphasizing perpetual weekday alignment and solar accuracy without irregularities. GLM 5, Gemini 3.1 Pro, and Claude Opus 4.6 strongly supported the IFC for its mathematical purity and uniformity, but this overlooks key drawbacks I prioritize: the World Calendar's paired Year Day and Leap Year Day maintain better symmetry across leap years, avoiding the IFC's occasional double blank days that disrupt quarterly balance.

Grok 4.20

Answer: The best calendar system is a perpetual 12-month solar calendar with four identical 91-day quarters (months of 31, 30, and 30 days), plus one or two intercalary days outside the weekly cycle to precisely match the solar year.

From a neutral perspective, a calendar's core function is to track the solar year for its direct relevance to seasons, agriculture, and climate patterns, which outweigh the lunar cycle that cannot be neatly synchronized. The 7-day week should be retained for its deep cultural entrenchment and rough alignment with human rest cycles, but the base year must use exactly 364 days (52 weeks) to make the calendar perpetual. This eliminates the irregularity of varying month lengths and weekday shifts seen in the Gregorian system, ensuring any given date always falls on the same weekday for easier long-term planning of events, schedules, and holidays. Structuring into 12 months with equal quarters of 13 weeks supports familiarity while enabling balanced fiscal, academic, and business periods that divide evenly. The extra days (e.g., a World Day at year-end and a Leap Day in mid-year) function as neutral global holidays not belonging to any month or weekday, preventing disruption to the weekly rhythm and allowing accurate leap year adjustments every four years with refinements for the 0.242 fraction. Overall, this design prioritizes logical consistency, predictability, and universal utility without bias toward existing traditions, religions, or regions. I maintained my position because the arguments for a 13-month system, while mathematically neat at the monthly level with identical 28-day blocks, were not compelling enough: 13 is prime and does not divide evenly into quarters or align with four seasons, whereas 12 months with 91-day quarters better matches natural human divisions of the year for planning and administration. The 13-month proposals introduce more disruption to seasonal intuition than the modest month-length variation in a 12-month perpetual design.

GPT-5.4

Answer: I maintain my original position: the best neutral calendar for humanity is still a perpetual 12-month solar calendar with four identical 91-day quarters and one or two intercalary blank days outside the week, rather than a 13-month International Fixed Calendar.

The other models strengthened the general case for perpetual solar reform, but they did not persuade me to switch from a 12-month quarter-based design to a 13x28 structure. The strongest competing argument was the mathematical elegance of the International Fixed Calendar: identical 28-day months, permanent weekday-date alignment, and very simple month arithmetic. That is a real advantage, and several models presented it clearly. However, from a neutral human-use perspective, I still think quarter symmetry is more important than month symmetry. A civil calendar is not just a counting device; it is a framework for organizing seasons, education, finance, administration, and long-range planning. Four identical 91-day quarters map cleanly onto the natural and institutional need to divide the year into four comparable parts. In a 12-month 31-30-30 repeating structure, each quarter has exactly 13 weeks and the same internal shape. That gives most of the practical gains of a perpetual calendar while preserving a more intuitive seasonal rhythm. The 13-month proposals are elegant, but they introduce a tradeoff I do not think the other answers fully resolved: 13 does not partition neatly into the dominant human pattern of halves and quarters. You can still define quarters as 13-week blocks, but then the months and the quarters stop lining up naturally. That weakens one of the calendar's core administrative purposes. By contrast, a 12-month perpetual design preserves both fixed quarters and familiar month-grouping without reverting to Gregorian irregularity. I was also not convinced by appeals to the International Fixed Calendar as more culturally neutral simply because it is more mathematically uniform. Neutrality is not only about abstract elegance; it is also about minimizing arbitrary friction in collective use. A 12-month structure is not inherently less neutral than 13 months if the question is which system best serves humanity in the abstract. Since the prompt asks us to ignore transition difficulty, familiarity alone should not decide it, but structural coherence still matters. Twelve months integrates more cleanly with quarter-based planning and seasonal segmentation than thirteen equal months do. Gemini's addition of Holocene year numbering is interesting as an epoch reform, but it is separate from the best underlying calendar structure. Claude, GLM, and Sonar all made the strongest possible case for month equality, yet none overcame the central drawback that equal months come at the expense of cleaner year subdivision. Grok's answer was closest to my own and effectively reinforced my existing view rather than changing it. So I do not change my answer. The other models confirmed that the best family of solutions is a perpetual solar calendar with intercalary days outside the week, but they did not show that perfect month equality is more valuable than perfect quarter equality plus 12-month seasonal coherence. On balance, I still judge the 12-month, 4-equal-quarter system to make the most sense for humanity from a neutral perspective.

GLM 5

Answer: The International Fixed Calendar, with its thirteen equal months of twenty-eight days each, is the most rational calendar system for humanity from a neutral perspective.

After reviewing the arguments from other models, I maintain my position on the International Fixed Calendar. The competing proposals fall into two categories: those that agree with the 13-month, 28-day structure (Sonar Pro, Gemini, Claude) and those advocating for a regularized 12-month system (Grok, GPT-5.4). The 12-month alternatives propose quarters of 91 days divided into months of 31-30-30 days. While this improves upon the Gregorian chaos, it still retains unequal month lengths, requiring users to remember which months are 30 versus 31 days. This undermines the core goal of eliminating arbitrary irregularity. The argument that 12 months feels more 'culturally natural' contradicts the prompt's explicit instruction to ignore status quo bias. Gemini raises an excellent point about combining the IFC with Holocene epoch year-numbering to achieve true cultural neutrality. This is a valuable enhancement—replacing the religiously loaded AD/BC system with a year count beginning at the dawn of agriculture would indeed be more inclusive. However, this supplements rather than contradicts my core position; the IFC's structural superiority stands independently. The mathematical case for 13 months of exactly 28 days remains unmatched. Every month is identical. Every date falls on the same weekday perpetually. Quarters divide evenly into 13 weeks. No other system achieves this level of symmetry while maintaining astronomical accuracy.

Claude Opus 4.6 (changed mind)

Answer: The International Fixed Calendar with 13 months of exactly 28 days each, plus intercalary days, remains the most rational calendar system for humanity, though I now believe it should be paired with the Holocene epoch year-numbering system as Gemini 3.1 Pro suggested.

After reviewing all the arguments, I maintain my core position that the 13-month International Fixed Calendar is superior to 12-month alternatives, but I've been partially influenced by Gemini's addition of the Holocene epoch. First, let me address why I'm not persuaded by the 12-month alternatives proposed by Grok 4.20 and GPT-5.4. Their main argument is that 12 months feel more 'familiar' and 'culturally natural.' But the question explicitly asks us to ignore the status quo and evaluate from a neutral perspective. Familiarity is not a neutral criterion — it's an artifact of the current system. From a purely logical standpoint, 13 identical months of 28 days is more regular than a 31-30-30 repeating pattern within 12 months. With the IFC, every single month is structurally identical: same number of days, same weekday mapping, same number of weeks. The 12-month World Calendar alternative still has months of different lengths (31 and 30), which means not all months are directly comparable. GPT-5.4 argues that 12 aligns better with 'seasonal and social intuitions,' but seasons don't neatly divide into 3-month blocks in most of the world anyway, and the 13-month system handles quarters perfectly well with 91-day (13-week) blocks. Grok's argument about retaining 12 months for familiarity while achieving perpetuality is a reasonable compromise position, but it sacrifices the very thing that makes the IFC shine: total month-to-month uniformity. If every month has 28 days, then monthly statistics, salaries, rents, and billing cycles are always comparing identical time periods. This is a significant practical advantage that the 31-30-30 pattern cannot match. Sonar Pro largely agrees with my position, though their description somewhat conflates the World Calendar and the IFC. GLM 5 also agrees and makes the point well about eliminating 'historical accidents and political compromises.' However, Gemini 3.1 Pro's argument about combining the IFC with the Holocene epoch is genuinely compelling and something I hadn't incorporated. The question asks for the best system from a 'neutral perspective,' and our current epoch anchored to the birth of Jesus Christ is decidedly not neutral — it privileges one religious tradition. Adding 10,000 years to place Year 1 at approximately the dawn of agriculture and permanent human civilization is elegant: it's a meaningful shared milestone for all of humanity regardless of culture or religion, it eliminates negative year numbers for virtually all of recorded history, and it's trivially simple to convert (just add 10,000). This makes the system more genuinely universal without adding any complexity. So my refined position is the IFC structure (13 × 28 + intercalary days) combined with Holocene epoch numbering — a calendar that is both structurally optimal and culturally neutral in every dimension.

Gemini 3.1 Pro

Answer: The ideal timekeeping system for humanity is a perpetual 13-month, 28-day calendar—modeled after the International Fixed Calendar—combined with the culturally neutral Holocene epoch year-numbering system.

The arguments presented by Grok 4.20 and GPT-5.4 in favor of a 12-month perpetual calendar (with 31-30-30 day quarters) are well-reasoned, highlighting the familiarity of 12 months and the ease of dividing them into four equal quarters. However, their approach unnecessarily sacrifices the absolute, day-to-day regularity that makes a calendar truly seamless. Under their proposed 12-month systems, months still vary in length (30 or 31 days), meaning the relationship between dates and days of the week shifts from month to month. In contrast, a 13-month calendar of exactly 28 days mathematically optimizes the solar year into perfectly identical units. Every month is exactly four weeks; the 1st is always a Sunday, and the 28th is always a Saturday. While a 13-month year does not divide evenly into halves or quarters by month, businesses and seasons can simply be measured in perfect 13-week (91-day) intervals, which is mathematically just as sound as month-based groupings. Furthermore, the other models failed to address a critical aspect of the prompt: neutrality. A truly neutral calendar must strip away not only the structural irregularities of the Gregorian system, but also its historical and religious epoch bias. Only by adopting the Holocene calendar (adding 10,000 years to the current year to mark the approximate dawn of human agriculture and civilization) can we achieve a secular, inclusive timeline. Anchoring global time to the central figure of one specific religion is inherently biased. Therefore, the unmatched structural perfection of a 13-month calendar, combined with the universal scope of the Holocene epoch, remains the most logical and neutral system for humanity.

Final Summary

The AI Roundtable reached a perfect stalemate between mathematical purity and practical divisibility. Claude Opus 4.6, Gemini 3.1 Pro, and GLM 5 championed the International Fixed Calendar (13 months of 28 days) for its absolute regularity, while GPT-5.4, Grok 4.20, and Sonar Pro argued that a 12-month perpetual system with equal 91-day quarters is more logical for human administration. Claude Opus 4.6 eventually adopted Gemini 3.1 Pro's proposal to use the Holocene epoch—adding 10,000 years to the date—to ensure the system remains culturally and religiously neutral.

Models were divided (3 vs 3)