BEHIND OUR CALENDARS AND CLOCKS

PART OF SERIES, "TRACKING TIME: BEHIND THE CLOCK AND CALENDAR"

Author: Ken Klark L. Flores

Photo: timeanddate.com; Raindrops & Roses ♥/Pinterest

Introduction

Clocks and Calendars are part of our daily lives, our hour is equivalent to 60 minutes, our day is equivalent to 24 hours, our weeks are 7 days, and 365-366 days are equivalent to one year. But why we have clocks and calendars, what's the basis of these things?

Day/Night and Calendar Cycle

In the biblical book of Genesis, the day/night cycle is an intentional part of God's creation, established from the very beginning. In the Day One, God created light and separated it from the darkness. He called the light "Day" and the darkness "Night". The text states, "And there was evening, and there was morning—the first day". This established the pattern for timekeeping, with a day beginning in the evening (darkness first, then light), a practice still observed in the Hebrew calendar (Why is a day measured from evening to morning in Genesis 1?, n.d.)

The day 2-3, this same pattern of "evening and morning" is repeated for the subsequent days, even though the sun had not yet been created. In the Day 4, God created the "two great lights" (the sun and moon) and the stars to provide consistent sources of light, to govern the day and night, and to serve as markers for seasons, days, and years. The existence of light and a day/night cycle before the creation of the sun on Day Four is a point of theological discussion, with some interpretations suggesting a temporary light source or a non-literal understanding of the "days" (Lacey, T., 2025)

 A "solar day" (what we typically refer to as a day) is about 24 hours. A "sidereal day" is the time it takes for the Earth to rotate exactly once, which is about 23 hours and 56 minutes (Erickson, 2021)

Circardian Rhythm

The 24-hour cycle is also regulated by the body's internal 24-hour clock, called the circadian rhythm. This internal clock regulates sleep-wake cycles and other important processes by responding to environmental cues, primarily light. A healthy, consistent circadian rhythm is crucial for overall health and well-being. Factors like irregular schedules, travel, or screen time can disrupt it (Reddy et al., 2023).

Tides and Temperature

Coastal areas experience two high tides and two low tides approximately every 24 hours and 50 minutes, due to Earth's rotation through two tidal bulges caused by the moon (National Oceanic and Atmospheric Administration, 2019). The diurnal cycle includes fluctuations in surface temperature throughout the day and night, mainly driven by solar radiation. 

Calendar

The observation of natural cycles—specifically seasons, days, and months—forms the core basis for all calendar systems developed throughout history. Days are founded on the Earth's rotation on its axis Months are historically derived from the lunar cycle (the orbit of the Moon around the Earth). Seasons and the year (approximately 365.25 days) are determined by the Earth's orbit around the Sun and the tilt of its axis.

Different cultures emphasized these elements in various ways, leading to solar calendars (like the Gregorian), lunar calendars (like the Islamic Hijri calendar), or lunisolar calendars (like the Hebrew calendar and Chinese Calendar), each structuring time based on these fundamental astronomical realities (Coin, 2016).

Accuracy

Clock accuracy measures how closely a clock's time matches the true passage of time, often expressed as a deviation per unit of time, like seconds per year. Factors like the quality of the oscillator crystal, temperature fluctuations, and assembly affect accuracy. While accuracy is how close a clock is to the correct time, precision is how consistently it measures time, or the fineness of its increments. (What Clock Error Means to Your Measurement System, 2025). The examples are typical quartz watch may be off by about 0.26 seconds per day, a very small amount over a large number of seconds, and Atomic Clocks are the most accurate timekeepers, with the most advanced atomic clocks being off by less than one second in millions of years. While it is the world's best experimental optical atomic clocks gain or lose a mere second every 40 billion years or so in ideal, isolated conditions. However, in reality, disturbances in the environment, even at a quantum level, mean some level of error always remains.

Time itself, in a physical sense, is a property of a path through spacetime, and even two "perfect" clocks will show different times if they travel different paths due to relativistic effects (Bouchard, 2020). The Earth's rotation is gradually slowing down due to the Moon's gravitational pull, making each day slightly longer than the last over the long term (by about 1.4 milliseconds per century). To keep our highly accurate atomic clocks in sync with the Earth's variable rotation-based time, "leap seconds" are occasionally added to the Coordinated Universal Time (UTC). This ensures that noon on a clock face aligns roughly with the sun's position overhead (How Long Is a Day on Earth?, n.d.)

Calendar accuracy varies, with the Persian calendar being the most accurate, followed by the Revised Julian calendar. The widely used Gregorian calendar is quite accurate, with an error of about 27 seconds per year, equivalent to one day in roughly 3,236 years. Other calendars have larger inaccuracies; for example, the Julian calendar drifts by a day every 128 years  (Welle (www.dw.com), n.d.).

Like humans, no calendars are perfect, as the astronomical year (the time it takes for the Earth to orbit the sun) is approximately 365.24217 days. Calendars use whole days, so they must have a system of adjustments, like leap years, to stay aligned with the solar year. The Gregorian calendar's rules for leap years are very accurate but still have a small, cumulative error. (The Revised Julian Calendar, n.d.).

The same thing goes for Clocks, a "perfect clock" is theoretically impossible to build due to the messy nature of reality and the fundamental laws of physics, and the concept of an exact 24-hour day is actually a human construct. In essence, clocks aren't perfect, and the natural "day" they are meant to measure isn't perfectly consistent either.

Reference

Lacey, T. (2025, November 12). Days Without a Sun? Answers in Genesis; Answers In Genesis. https://answersingenesis.org/days-of-creation/days-without-sun/?srsltid=AfmBOoovKewhDKDSMSNIvyi28NEJTfKY4L4SsPbcoZ7iErPspo48SFEJ

Why is a day measured from evening to morning in Genesis 1? (n.d.). GotQuestions.org. https://www.gotquestions.org/evening-morning-Genesis.html

Erickson, K. (2021, February 9). How long is one day on other planets? | NASA Space Place – NASA Science for Kids. Nasa.gov. https://spaceplace.nasa.gov/days/en/

Reddy, S., Sharma, S., & Reddy, V. (2023). Physiology, Circadian Rhythm. Nih.gov; StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK519507/

National Oceanic and Atmospheric Administration. (2019). Frequency of Tides - The Lunar Day - Tides and water levels: NOAA’s National Ocean Service Education. Noaa.gov. https://oceanservice.noaa.gov/education/tutorial_tides/tides05_lunarday.html

‌Coin, G. (2016, February 29). Why is it called “leap” year? 5 fun facts to make you smarter on Leap Day (video). Syracuse. https://www.syracuse.com/news/2016/02/why_do_we_call_it_leap_year_fun_facts_to_make_you_smarter_on_leap_day_video.html

What Clock Error Means to Your Measurement System. (2025). Ni.com. https://www.ni.com/en/support/documentation/supplemental/06/what-clock-error-means-to-your-measurement-system.html

Welle (www.dw.com), D. (n.d.). No perfect calendar: Why we have leap years and why the Earth’s always out of sync | DW | 28.02.2020. DW.COM. https://www.dw.com/en/no-perfect-calendar-why-we-have-leap-years-and-why-the-earths-always-out-of-sync/a-19077064

The Revised Julian Calendar. (n.d.). Www.timeanddate.com. https://www.timeanddate.com/calendar/revised-julian-calendar.html

Bouchard, R. P. (2020, January 24). A Day Is Not 24 Hours. Medium. https://medium.com/the-philipendium/a-day-is-not-24-hours-c36ee96078c6

How Long Is a Day on Earth? (n.d.). Www.timeanddate.com. https://www.timeanddate.com/time/earth-rotation.html

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