When we glance at a clock or check the date on our phones, we often assume that timekeeping is a fixed and universal system. But the way humans track time is anything but standard. Time may be universal, but the way we track and define it is not. Across cultures and centuries, civilisations have developed vastly different calendars. While today’s globally dominant Gregorian calendar has become a default in much of the world, it’s far from the only way to count the days.
The science behind calendars is deeply intertwined with astronomy, mathematics and cultural priorities. It reflects not only how we observe the sky but also how we choose to structure our lives around those patterns. Understanding how and why different calendars exist sheds light on the intersection between science and culture and why time isn’t as objective as it first appears.
The Different Types of Timekeeping
Most calendar systems are based on the movement of celestial bodies. The Gregorian calendar, used widely in the West, is a solar calendar. It tracks the Earth’s orbit around the sun, organising the year into 365 days, with leap years added every four years to correct for orbital inconsistencies. This system is designed for agricultural societies that rely on solar patterns to determine planting and harvesting seasons.
By contrast, the Islamic calendar is purely lunar. It is based on the moon’s phases, with months beginning when the new crescent moon is first sighted. A lunar year has about 354 days, meaning Islamic holidays move through the seasons over time. This is why observances such as Ramadan and Muharram in 2025 fall on different dates than they did in 2024 when compared against the Gregorian calendar. In 20 (Gregorian) years’ time Muharram will go from being in the summer months to being in the middle of winter, with all the other Islamic months moving likewise. This can have big impacts on occasions such as the month of Ramadan when Muslims fast during daylight hours –a period that is much longer in the summer than in the winter.
Some cultures have developed lunisolar calendars to reconcile lunar months with the solar year. The Chinese calendar, for instance, incorporates both lunar phases and solar positions, adding a thirteenth leap month every few years to stay in alignment with the seasons. This hybrid system ensures that festivals such as Chinese New Year always occur in late winter or early spring, rather than drifting throughout the year.
Calendars as Cultural Code
The design of a calendar often reveals the priorities and values of the culture that created it. Agricultural societies needed to align their calendars with the sun to ensure successful crop cycles. Nomadic or religious communities, on the other hand, might focus on the moon’s phases to structure rituals or migrations. Over time, calendars became cultural codes. By regulating traditions, festivals and social rhythms into a repeatable annual structure, they become part of them. Often such a long-standing and integral part of them that we don’t really notice. But step back and think how thoroughly the divisions and dates of the familiar Gregorian calendar influence how we perceive the passage of time and the key events of our year.
This cultural coding is still present today. Many countries officially observe national or religious holidays based on non-Gregorian calendars, requiring complex date conversion for international recognition. In India, for example, multiple calendars coexist across different regions and religions. Meanwhile, in Israel, the Hebrew calendar determines the timing of Jewish holidays, while the Gregorian calendar governs civil affairs.
The Scientific Legacy of Timekeeping
Despite their cultural differences, all calendar systems rely on empirical observation. Early astronomers across Mesopotamia, Mesoamerica, China and the Islamic world all engaged in systematic tracking of celestial bodies. Their observations led to remarkably precise estimates of lunar cycles and solar years long before modern tools existed. In fact, many of the astronomical tables and mathematical models developed in service of timekeeping laid the groundwork for broader scientific discovery.
Modern science continues to refine our understanding of time. Today, atomic clocks track time with unparalleled accuracy, measuring the vibrations of caesium atoms rather than the movement of the sun or moon. Yet, even as science offers increasingly precise tools, the calendars we use still reflect deeply human concerns.
Time, Science and Human Stories
The diversity of calendars can often be seen as a complication. But it’s also a reminder that time, though rooted in the cosmos, is interpreted through human lenses. Whether one is following school terms, fiscal quarters, lunar festivals, or agricultural seasons, every calendar tells a story.
