Format lists the day first, followed by the month and then the year. For example, 22 January 2026 is written as "22 January 2026" or "22/01/2026". This format is natural and widely used in Europe, Africa, and much of Asia.

Places the month first, then the day, and finally the year. For example, January 22, 2026, appears as "January 22, 2026" or "01/22/2026". This format is most widely used in the United States and emphasizes the month. However, when both day and month are numbered 12 or lower, this format may cause international confusion because the order can be misunderstood.

Format starts with the year, followed by the month, and finally the day. For example, 2026-01-22 is written as "2026-01-22". This is the most logical and technically friendly structure, moving from the largest time unit to the smallest. It's widely used in East Asia and serves as the basis for international technical standards.

A calendar date represents a specific day on a calendar, defined by year, month, and day. For example: 22 January 2026. This is the most common and intuitive type of date, answering the simple question: "Which day is it?" Calendar dates are used for birthdays, appointments, public holidays, and daily records. They don't include time and are treated as a full-day reference.

A date-only value represents a calendar date (such as 2026-01-22) without any time information. This is commonly used in databases (systems for storing data) and forms (electronic or paper documents for data entry) when the exact time is irrelevant. Using a date-only value prevents errors from accidentally including time zones (regional time differences) or time-of-day details. Typical uses include birthdates, due dates, expiration dates, and schedules for school or work.

A date-time combines a calendar date (for example, year, month, and day) with a specific time of day (such as hours, minutes, and seconds), for example: 2026-01-22 14:30:00. It answers: "Exactly when did this happen?" Date-time values are crucial for event scheduling, system logs, financial transactions, and booking systems. They provide precision to the level of hours, minutes, seconds, and, in some cases, milliseconds (thousandths of a second).

A timestamp marks the precise instant an event occurs. It is a detailed, system-specific date-time value, often stored as a numeric value relative to a fixed reference point (called an epoch). The format or implementation can vary depending on the computer or software used. An epoch is a fixed starting date and time from which timestamps are counted. Timestamps ensure uniformity across systems, including those in different time zones, and are optimized for readability, precision, accuracy, and retrieval of exact moments.

A start date marks the beginning of an event, process, or period. For example: Employment start date, and project start date. It answers the question: "When did this begin?" Start dates are essential for contracts, subscriptions, projects, and academic terms. They often work together with end dates to define a range.

An end date represents the conclusion of an activity, agreement, or time period. Common examples are contract end dates and membership expiration dates. It answers: "When does this stop being valid?" End dates are important for legal clarity, billing cycles, access control, planning, and renewals.

A due date defines the final acceptable date by which an action must be completed. For example: assignment due date or payment due date. A due date signals accountability and urgency. Missing it can result in penalties, delays, or complications.

An expiration date indicates when something becomes invalid or unusable, such as the expiry of a product, credit card, or license. This date is critical for safety, compliance, security, and quality control. After this date, the item or permission is no longer valid or reliable.

An effective date specifies when an action, agreement, or period becomes valid. Examples include the contract effective date, membership start date, and license effective date. This is key for legal clarity, billing, access, planning, and renewals.

A duration specifies the length of time an action, agreement, or period lasts. Common examples include project, membership, and license durations. Duration is critical for legal precision, billing, access, planning, and renewals.

A record date (or entry or posting date) is the technical term for when information is officially entered into a system or document. It does not always match the date the event happened. For example: A transaction record date or a medical record entry date. Separating record dates from event dates is important for audits and tracking.

An event date is the specific day on which an event occurs. For example, wedding date, conference date, and election date. Event dates can be date-only or date-time, based on the needed detail.

A relative date is defined in relation to another date rather than as a fixed calendar value. Common examples are: Today, tomorrow, 30 days from now, and last week. People use relative dates in reminders, filters, automation, and UIs because they change over time.

Solar calendars are designed around the Earth's orbit around the Sun, which we experience as a year and changing seasons. Their goal is to keep the calendar aligned with the seasons (summer remains summer, etc.). The solar year is approximately 365.2422 days, which isn't a whole number. Therefore, solar calendars use leap days to maintain accuracy. They're best suited for farming, civil planning, and ensuring seasonal consistency. A famous example is the Gregorian calendar, which is used most widely worldwide.

A lunar calendar follows the Moon's phases, typically beginning with a new moon or visible crescent. A lunar month is approximately 29.5 days, and twelve lunar months total about 354 days, which is roughly 11 days shorter than a solar year. Consequently, a purely lunar calendar gradually drifts through the seasons. Lunar calendars are best suited for religious observances tied to the Moon, and many systems feature months alternating between 29 and 30 days.

Lunisolar calendars strive to harmonise both the lunar and solar systems. They maintain lunar months while aligning years with the Sun's position (seasons). To address the discrepancy, lunisolar calendars introduce an extra month (leap month) in certain years. This system is ideal for societies seeking moon-based months but season-based years. It offers a clever compromise: "true" lunar months without letting festivals drift into different seasons.

To maintain alignment with the Sun, a year divisible by 4 is typically a leap year. However, years divisible by 100 are not leap years unless they are also divisible by 400. This balancing act prevents the calendar from gradually drifting away from seasons over centuries.

This lunar calendar follows the Moon's phases and has a shorter year than the solar year. Consequently, its months shift through the seasons over time. It's used for religious observances, fasting periods, and major holidays in Muslim communities.

The Hebrew calendar, like the lunisolar calendars of other cultures, uses lunar months but adds leap months to ensure major observances align with the seasons. Its core principle is to maintain lunar-based months while keeping seasonal alignment.

The Chinese calendar, traditionally used for festivals and cultural events, also employs lunar months with periodic leap months. Today, it's primarily used for Lunar New Year timing and traditional festivals.

Hindu calendars vary regionally and community-wise. Some are lunisolar and involve complex astronomical calculations. They're commonly used for religious festivals, auspicious dates, and ceremonial planning.

The Ethiopian calendar, a distinct solar-based system, has its own structure and numbering. It's officially used in Ethiopia alongside international systems in certain contexts.