The messy history behind the modern calendar

Summary

Podcast Summary: "The Messy History Behind the Modern Calendar"

Podcast: More or Less (BBC Radio 4)
Host: Tim Harford
Guest: Dr. Kristen Lippincott, former director of the Royal Observatory at Greenwich and author of The Story of Time
Date: June 27, 2026

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Episode Overview

This episode explores the history, logic, and arbitrariness of the modern calendar, sparked by a listener's question about why meteorological records are organized by calendar months—such as "the hottest May day." Tim Harford delves into how calendars were constructed, why months are the lengths they are, and the cultural, scientific, and emotional influences behind different systems of timekeeping. Dr. Kristen Lippincott provides expert insight, debunking the idea of a purely "rational" calendar and highlighting the messy, fascinating roots of how humanity has divided time.

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Key Discussion Points & Insights

The Significance and Arbitrary Nature of Months

• Prompt: Recent temperature records in the UK are reported in terms of calendar months, which listeners find arbitrary.
  • Tim Harford asks: “When meteorologists start quoting records in arbitrary periods, such as the month of May, wouldn't it be better to narrow down these periods to more manageable and equal bites?” (00:49)
• Kristen Lippincott responds: Months are arbitrary as just collections of days with historical names, but the need for months and calendars stretches back to primal survival needs—to track environmental changes and seasonal events.
  • "It is arbitrary in that May is just a name for a group of days put together. But the idea of May and the idea of having a seasonal calendar goes way, way, way back to prehistoric times." (01:24)

The Origin of Calendrical Systems

• Survival and the Sky:
  • Early people created calendars to track vital cycles: rains, animal migrations, migration-friendly sailing, floods, etc.
  • Example: The Egyptians linked the annual flooding of the Nile to the morning rising of Sirius. (01:44)
  • Example: Greeks used stellar events—“Hesiod says, don’t sail when the Pleiades have set.” (02:18)
• Written and Lunar Calendars:
  • The earliest calendars recorded lunar phases. Evidence includes scratchings on ancient bones.
  • “Most of the very early calendars are lunar. And then later you get the solar calendar.” (02:42)
  • Civilizations now use varied systems—some use only lunar calendars (e.g., Islamic), some a combination (e.g., Jewish calendar, Julian/Gregorian calendar).

From Lunar to Solar: Decoding Today’s Month Lengths

• Early months tracked the 29.5-day lunar cycle; adapting to fit the solar year, the months’ lengths became irregular.
  • “May has 31 days, but February usually has 28.” (03:39)
  • This was a result of the struggle to align lunar and solar cycles.

Diverse Calendars Across Civilizations

• Mesopotamians: Built on a base-60 system, influencing our hours/minutes/seconds.
  • “The Mesopotamians were the great mathematicians of antiquity…” (04:06)
• Egyptians: Used a base-10 system.
• Aztecs/Maya:
  • Had 20-day “weeks” and a ritual year of 260 days.
  • The 260-day cycle possibly links to human gestation, intervals between eclipses, or the cycle of Venus.
  • “Their year was a ritual year which was 260 days. … Most people think it has to do with Venus, which reappears as the morning star every 263 days.” (04:21)
• Key theme: Each culture tuned its calendar to fit its needs, cycles, and religious/cultural beliefs.
  • “Everybody's looking for things that suit their particular religions or yearly cycles so they're all different until civilizations bring them together…” (04:59)

The French Revolution’s Decimal Calendar

• Rational Experiment:
  • The French introduced a decimal system: 10-day weeks, 12 months of 30 days.
  • Tim: “This was an attempt to do what our listeners were talking about, to split the year into scientific, rational chunks.” (05:25)
• Kristen debunks the ‘rationality’:
  • “It was emotional. They wanted to get rid of the power of the Catholic Church so they had to get rid of the old calendar… Even though it looks numerically ... 'scientific', it's actually ... deeply emotional and culturally based.” (05:53)
  • The reform failed mainly due to incompatibility with neighboring countries’ calendars.

What Does Today’s Calendar Say About Us?

• Is our modern calendar meaningful, or just an artifact?
  • Kristen argues that our current system is less about ultimate precision and more about collective cultural, emotional, and religious inheritance.
  • “One could throw it out, have a decimal calendar or base all of our time on ticks of the cesium clock. But…our calendar is best able to accommodate the reality of where we live on Earth.” (06:59)

Precision vs. Practicality: Recording Temperature Records

• Is it “silly” or misleading to segment record temperatures by named months?
  • Tim: “Do you have sympathy with that complaint?” (07:39)
  • Kristen: “I wouldn't call it silly. ... It's not a question of precision equaling truth, it's a question of how we manage to approximate a seasonal, a yearly, a monthly cycle. And the best way is our quote unquote silly calendar.” (07:58)

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Notable Quotes & Memorable Moments

• On Arbitrariness:
  • “May is just a name for a group of days put together.” — Kristen Lippincott (01:24)
• On Early Calendars:
  • “The first reason why you would have a calendar is basically for survival.” — Kristen Lippincott (01:44)
  • “[People] could tie these, what we would now call annual events, to the positions of the stars...” (01:55)
• On the Messiness of Modern Timekeeping:
  • “It’s not a question of precision equaling truth, it’s a question of how we manage to approximate a seasonal, a yearly, a monthly cycle.” — Kristen Lippincott (07:58)
• On the Emotional Roots of ‘Scientific’ Calendars:
  • “Even though it looks numerically because it is decimal based, as if it's quote unquote scientific, it's actually like most calendars, deeply emotional and culturally based.” (05:53)
• Final Thought:
  • “Our calendar is best able to accommodate the reality of where we live on Earth.” — Kristen Lippincott (06:59)

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Important Timestamps

• 00:49 – Challenging the logic of calendar months for temperature records
• 01:24 – The arbitrary yet ancient nature of months
• 02:42 – On the first written calendars and lunar systems
• 04:21 – The workings of the Aztec/Maya ritual year
• 05:25 – The French Revolutionary calendar experiment
• 05:53 – Why “rational” calendars are still emotional and political
• 06:59 – What our calendar reveals about modern society
• 07:58 – Final word on precision and practical timekeeping

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Conclusion

The episode underscores that our way of marking time—months, years, historical records—is a patchwork of necessity, culture, and emotion, not scientific perfection. The modern calendar is, at its heart, a practical compromise reflecting humanity’s messy but beautiful adaptation to its environment, cultural history, and need for shared rhythms.

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For more insights or to question the numbers you hear, the More or Less team invites listener contributions.

Transcript

[A]

Hello and thanks for downloading the More Or Less podcast with a programme that looks at the numbers in the news, in life and in ancient Maya calendars. And I'm Tim Harford. Last month, the UK's Met Office recorded a temperature of 35.1 degrees Celsius from a sensor at London's Kew Gardens. This was, the press reported, the hottest May day since records began. A couple of our listeners got in touch to point something out. While 35.1 degrees seems like a sensible, rigorous way of measuring temperature, the month of May feels like an arbitrary way to record time.

[B]

This record was set quite late in the month on the 25th of May. That made me wonder whether calendar months are always the most meaningful way to compare temperatures.

[A]

When meteorologists start quoting records in arbitrary periods, such as the month of May, wouldn't it be better to narrow down these periods to more manageable and equal bites? So, months, eh? What's going on there? To find out more, I spoke to Kristin Lippincott, former director of the Royal Observatory at Greenwich and author of the Story of Time. Well, welcome to More Or Less. Thank you. Now, our listeners have asked if May, the month of May, is an arbitrary way to measure time. Is it?

[B]

Well, yes and no. It is arbitrary in that May is just a name for a group of days put together. But the idea of May and the idea of having a seasonal calendar goes way, way, way back to prehistoric times.

[A]

So what were these prehistoric calendars like?

[B]

The first reason why you would have a calendar is basically for survival. It would be tracking the changes in environment, in climate. For example, you'd want to know when the rains were coming, when it would be hot, when it would be cold, animal migration, sailing in the Mediterranean, things like that. People very quickly realized that they could tie these, what we would now call annual events to the positions of the stars in the skies. The so, for example, the annual flooding of the Nile came with the morning rising of Sirius, or Hesiod says, don't sail when the Pleiades have set. So it's kind of how you can set markers in order to keep your community surviving.

[A]

And presumably, if we know something about these early calendars, they must have been recorded in some way. So what are the first written calendars? What do they look like?

[B]

Well, the first written calendars are usually things like tracking the phases of the moon. There are scratchings on old bones that we now think are probably rudimentary calendars. If you think about yourself sitting in a field for a year, the first thing you will notice is how the moon changes every night. And that's why most of the very early calendars are lunar. And then later you get the solar calendar when they say, oh yeah, we've come full circle again and the sun's in the same place as it was in the sky, this time 365 days. So if you look, for example, some civilizations still use lunar calendars like Islam. Others use a lunar solar calendar, which is the Julian calendar or our calendar, but also the Jewish calendar. And so there's always this approximation between trying to get a cycle of a year to match the positions of the sun, the moon and the stars.

[A]

And it's an approximation that has left us with the calendar we have today. The month started by tracking the phases of the moon, about 29 and a half days. But as the calendar evolved to align with the solar year, the lengths shifted, which is why May has 31 days, but February usually has 28. So that's how we currently cut up the year. But how did other civilizations do it in the past?

[B]

Every single culture has its own calendar to suit its own needs. At the Mesopotamian calendar, the Mesopotamians were the great mathematicians of antiquity and they were the ones who set up the 60 base number series which we see

[A]

in our sort of hours, minutes, seconds.

[B]

Yes. And so we have inherited it. The Egyptians had a 10 base calendar, so we've inherited some from that. One of my favorites is the Aztec Mayan calendar, which is actually 20 day weeks. And their year was a ritual year which was 260 days. And some people have said it might be related to the amount of time it takes an infant to come to full term before it's born, which is nine months. It might be a period between two eclipses, which is about 260 days. Or most people think it has to do with Venus, which reappears as the morning star every 263 days. So everybody's looking for things that, that suit their particular religions or yearly cycles so they're all different until civilizations bring them together and say, no, we're gonna have this kind of calendar. And that's when people start to get really restrictive about how many days they are, how long they are, what the months are called.

[A]

Then comes an important moment in the history of calendars. The French Revolution. In their thirst for a rational reinvent of literally everything, they adopted a 10 day week and split the year into 1230 day months. This was an attempt to do what our listeners were talking about, to split the year into scientific, rational chunks.

[B]

Well, two words There that I would say don't quite fit with my thinking. Scientific and rational, basically. It was emotional. They wanted to get rid of the power of the Catholic Church so they had to get rid of the old calendar so they could get rid of the saints days. So they said let's return to a bucolic agricultural colander and we'll call it Today is the day for growing grass or Today is the day for picking grapes. Even though it looks numerically because it is decimal based, as if it's quote unquote scientific, it's actually like most calendars, deeply emotional and culturally based.

[A]

And it didn't last.

[B]

No, because they were surrounded by countries who didn't keep them that calendar. And it became really, really difficult when you're having any kind of international negotiations.

[A]

If you have all of these historical calendars that reflect different values and different priorities. What should we conclude about the modern calendar? Does it tell us anything about what we value today or is it just a historical throwback?

[B]

Well, historical throwback again, I wouldn't say that our calendar is based on shared cultural, emotional, religious assumptions and inheritances. One could throw it out, have a decimal calendar or base all of our time on ticks of the cesium clock. But the truth of the matter is it works with the fact that we are on a rotating and revolving rock that happens to go around the sun, that happens to have a moon that goes around us and. And our calendar is best able to accommodate the reality of where we live on Earth.

[A]

Our listeners have said that it is slightly silly or at least arbitrary to quote a temperature record based on the fact that this is the hottest day that we've ever had in the UK in the month of May. Do you have sympathy with that complaint?

[B]

I wouldn't call it silly. I think the problem is, is when one thinks of any of the ways we divide and mark time as having an ultimate reality, it doesn't. It's not a question of precision equaling truth, it's a question of how we manage to approximate a seasonal, a yearly, a monthly cycle. And the best way is our quote unquote silly calendar.

[A]

So there we go. Yes, the month of May is arbitrary and our very, very hot June likewise. But then so are all calendars, it seems. Our thanks to Kristen Lippincott, former director of the Royal Observatory at Greenwich and author of the Story of Time. If you've seen a number you think we should take a look at, email us at more or lessbc.co UK until next time. Goodbye.