The Rest Is Science – "What Day Is It, Really?"

Podcast: The Rest Is Science
Hosts: Professor Hannah Fry & Michael Stevens (Vsauce)
Episode Date: December 30, 2025

Episode Overview

This episode unpacks one of the most fundamental yet surprisingly complex aspects of human life: timekeeping. Hannah Fry and Michael Stevens question our collective assumption that we truly know what day, year, or even era it is, and explore the fraught, messy, and sometimes downright strange history of how humanity has counted days, measured years, and standardized time. From the "missing days" of calendar reform to debates over the true year, phantom centuries, and atomic clocks, the hosts trace what we mean by “today”—and why that question is much weirder than it sounds.

Key Discussion Points & Insights

1. Questioning Reality: What Day Is It?

• [01:28] Michael jokes about whether it's really the new year, launching the episode’s theme: Are we really sure what time it is?
  • “Is it really a new year? Is it really 2026? Like, are we sure people have been counting without missing… a day or a year here or there?” (Michael Stevens, 01:31)
• [01:46] Hannah and Michael agree to explore "days that never happened" and centuries that disappeared.

2. How Calendars Get It Wrong (& Then Try to Fix It)

• The Julian Calendar’s Flaw

  • [02:39] Michael: The Julian calendar assumed 365.25 days/year.
  • [03:12] Hannah: "That is out for how long it takes for the Earth to actually rotate around the sun by 11 minutes and 14 seconds every single year."
  • Errors compounded, throwing off the date of the equinox.

• Religious Stakes for Calendar Reform

  • [04:17] Hannah: Easter’s timing depended on the spring equinox. The slip caused real world frustration, including Roger Bacon’s plea to the Pope: "Eventually Easter is going to be a summer holiday!"

• Fixing the Date: Gregorian Calendar

  • [07:06] Michael: Pope Gregory XIII “just jump[ed] 10 days forward,” introducing a new leap year system.
  • Leap Year Rule [[07:27]]: "If a year is divisible by 100, it’s not a leap year unless it’s also divisible by 400."

3. Centuries That “Never Happened”: The Phantom Time Hypothesis

• [14:22] Michael introduces Heribert Illig’s “Phantom Time Hypothesis.”
  • "The idea is that the Pope was off because the Popes had lied to us about what year it was, that there were three missing centuries, which accounts for those three days that they didn't have to add on." (Michael, 14:22)
  • Claims 614 to 911 CE never happened; Charlemagne was "essentially fan fiction."
  • [16:15] Hannah quips: "Charlemagne’s essentially fan fiction? Is that what we’re saying?"
• [17:22] Why it’s easily debunked:
  • Dendrochronology: Tree ring records align perfectly with written historical records, showing no missing centuries.
  • [19:28] Hannah explains, “You can daisy chain this so that we have basically an unending record ... that goes back over 10,000 years.”
  • Astronomical Events: Solar eclipses and a massive volcanic eruption in 536 A.D. are anchored in both history and nature’s records.

4. Behind "New Year’s Day": Arbitrary Beginnings

• [25:50] Michael is baffled: "I actually have no idea why January 1st was chosen to be the beginning of the year."
• [25:56] Hannah: Origins are Roman bureaucracy and convenience—original Roman calendar started with March, but shifted to January for administrative purposes.
  • "Basically, the reason why it's the 1st of January? It's admin. Admin is the father of this modern holiday." (Hannah, 28:04)
• Inconsistent Adoption: Not all nations or denominations agreed on the calendar right away. As late as the 1750s, countries still celebrated New Year on different days.

5. How Knowing the Time Saves Lives (and Sometimes Kills)

• [29:10] Coordinating time became essential for navigation (longitude) and later, for railways.
  • Longitude at sea: Sailors needed to know the precise time in London while at sea to determine their east-west position. Clocks would drift, leading to disasters:
    • "These sailors died because they were in the wrong place, because they didn't know what time it was?" (Michael, 32:03)
    • "Exactly. Thousands of people in this massive disaster." (Hannah, 32:13)
  • The push for accurate marine chronometers becomes a major scientific challenge going into the 18th century.
• Trains & Standardized Time: Trains made local time impractical, leading to standardized time zones.

6. Atomic Clocks: Replacing Earth with Atoms

• [35:09] Atomic clocks replace the sun and Earth's movements as the reference for timekeeping.

• [36:01] Hannah explains:

  • "If you give an atom of cesium exactly the right amount of energy, then its electrons will jump between energy levels at this frequency that is absolutely specific … 9,192,631,770 oscillations per second."
  • Current atomic clocks are precise to losing less than a second in 15 billion years.

• [39:53] Negotiating international time still involves a human touch: "Once a month, all of that data gets collected by a group in Paris ... but they communicate via email. So the way that we know what time it is is that we do it by email." (Hannah)

• Leap Seconds: Added to correct for the Earth’s irregular rotation.

7. The Universe Refuses to Cooperate: Universal Time?

• [41:14] As we look to Mars and interplanetary living, time becomes even trickier:
  • "A day on Mars … is not 24 hours. It's like 24 hours and then another 36 minutes."
• Earth's Rotation Slows Over Time
  • Ancient coral shows days used to be shorter; Earth is gradually spinning slower, so the number of days per year has changed across history.
    • "400 million years ago, there were 420 days in the year." (Hannah, 43:03)
  • “Time is both very precise as a concept, but also when it comes to living in time … it's like smoke escaping through our fingers.” (Michael, 44:38)

8. Time, Thyme, and Letting Go

• [44:38] Michael plays with etymology, linking "time" (dividing events) and “thyme” the herb, which may come from a word meaning “smoke.”
  • "Maybe time has been time all along. Just this smoky essence that you can't quite grab onto, but you keep trying to hold on to." (Michael, 45:49)
• [46:01] Hannah closes:
  "Stop trying to hold onto the smoke. Everybody just… Just let it go."

Notable Quotes & Memorable Moments

• On the Julian Calendar’s error:
  • “Burned. Oh, Julius, you idiot.” (Michael, 03:22)
• On the "missing" days of 1582:
  • “If your birthday's on the 10th, isn't that unlucky?” (Michael, 08:04)
  • “I always say, oh, I think that happened on October 7, 1582 ... they look it up and they go, there never was a seventh of October 1582. He’s a wizard. And I’m like, guilty.” (Michael, 08:07)
• On religious suspicion:
  • "Protestants ... thought actually this was a trick that the Catholic Church were pulling to make it so the Protestants celebrated Easter on the wrong day and therefore would damn their souls to hell." (Hannah, 08:59)
• On Phantom Time:
  • “Charlemagne’s essentially fan fiction? Is that what we’re saying?” (Hannah, 16:15)
  • “That means Charlemagne never existed. That means that the Carolingian period was a complete fabrication.” (Michael, 15:57)
• On atomic clock precision:
  • "These things are so phenomenally accurate that they would lose less than a second over 15 billion years." (Hannah, 37:48)
• On the futility of absolute time:
  • “Time is both very precise as a concept, but also when it comes to living in time … it's like smoke escaping through our fingers.” (Michael, 44:38)

Timestamps for Key Segments

• 01:28 | "Is it really a new year?" – Kicking off the big question
• 02:39–07:49 | Julian vs. Gregorian calendar: counting errors, leap years, calendar reform
• 14:22–21:31 | The Phantom Time Hypothesis and its debunking
• 25:56–28:04 | Why January 1st? Roman admin and moving New Year’s Day
• 29:10–34:58 | Navigating by time: longitude, clocks, and disasters at sea
• 35:07–39:53 | Atomic timekeeping explained (cesium clocks, international sync)
• 41:03–43:03 | Leap seconds, Earth's slowing rotation, and coral timekeeping
• 44:38–46:01 | Etymology of time/thyme and philosophical closing

Tone & Language

The hosts are witty, skeptical, and playful throughout, blending science, history, curiosity, and a sense of existential wonder. Their style keeps complex ideas approachable and makes the episode both educational and entertaining.

Takeaway

The calendar and clock are human attempts to tame the shifting, complex flow of time—the deeper you dig, the more arbitrary, contentious, and fragile the very idea of “what day it is” turns out to be. Whether it’s skipping days, losing track of centuries, or arguing over atomic clocks, reality keeps slipping through our fingers—like smoke.

“Stop trying to hold onto the smoke. Everybody just... let it go.” (Hannah Fry, 46:01)

For anyone wanting a mind-bender about the secret weirdness of time, this episode is essential listening.