Calendars are divided into smaller units of time to track the passage of days, months, and years more precisely. These subdivisions allow for more accurate measurement of time and are used in various combinations in different calendars to reflect the needs and cultural practices of their respective societies.

Days are the most basic subdivision of calendars and are defined as the time it takes for the Earth to rotate once on its axis. In most calendars, days are grouped into weeks, which are typically seven days long. The names of the days of the week often reflect cultural and religious influences. For example, the days of the week in English are named after Norse gods, while in many Romance languages they are named after celestial bodies.

Months are another subdivision of calendars, typically based on the cycles of the Moon. Lunar calendars have months of either 29 or 30 days, while solar calendars have months that are either 30 or 31 days long. In the Gregorian calendar, months have between 28 and 31 days, except for February, which has 28 days except in leap years when it has 29.

Hours are a further subdivision of time, typically used in combination with the day and month. In the Gregorian calendar, a day is divided into 24 hours, each of which is divided into 60 minutes and each minute into 60 seconds. Other calendars, such as the ancient Egyptian calendar, used a different system of hours, dividing the day and night into 12 parts each.

In conclusion, the subdivisions of calendars play a critical role in tracking the passage of time and organizing daily life. These subdivisions reflect the diverse needs and cultural practices of different societies and are used in various combinations to create the many different systems of calendars that have been developed throughout history.