Ajanlaskun

Ajanlasku is the system by which societies measure, arrange and name time. It encompasses the division of the day into hours, minutes, and seconds, the division of the year into months and days, and the broader conventions for counting years and eras. It also covers the selection and use of calendar systems that synchronize civil time with astronomical cycles.

Calendars and eras vary by culture. Solar calendars are centered on the solar year; lunar calendars are based on lunar months; lunisolar calendars combine both. The most widely used civil calendar today is the Gregorian calendar, a solar calendar introduced in 1582 to reform the Julian calendar’s drift. Many regions adopted it over subsequent centuries. Other calendars—such as lunar Islamic, Hebrew, Hindu, and traditional East Asian calendars—are still used for religious, ceremonial, and cultural purposes, often alongside the Gregorian system. Years may be counted in various era schemes, such as CE/BCE or regnal or era names in different traditions.

Timekeeping also involves standards and devices. The second is defined in terms of atomic transitions; Coordinated Universal Time (UTC) and time zones govern civil time. Leap seconds are occasionally added to keep UTC in line with Earth's rotation. In computing and science, time is commonly represented as epoch time, the number of seconds since a fixed point such as 1970-01-01 UTC in Unix-like systems.

In Finnish usage, ajanlasku refers to the study and description of these methods of measuring and organizing time. It forms a foundational concept in history, astronomy, and everyday life.