Datekeeping

Datekeeping is the practice of recording, organizing, and standardizing dates to establish chronology. It encompasses calendar systems, date notation, and the integration of dates with timekeeping for planning, historical record-keeping, and research. Across cultures, datekeeping serves civil, religious, scholarly, and archival purposes.

Calendars are the primary tools of datekeeping. Solar calendars, such as the Gregorian calendar used in most of the world today, align with the tropical year and include leap years to keep dates in line with seasons. Lunisolar calendars, used by several cultures, add months to accommodate lunar cycles within a solar year. Pure lunar calendars, such as the Islamic calendar, do not keep solar alignment. Many calendars also employ era designations, like BCE/CE or BC/AD, and some use regnal years or other epoch systems.

Date notation varies by region and institution. Common formats include day-month-year, month-day-year, and year-month-day; international standards such as ISO 8601 specify YYYY-MM-DD to avoid ambiguity. In archival and technical contexts, dates are often serialized or encoded as ordinal dates, Julian day numbers, or Unix timestamps (seconds since 1970-01-01 UTC).

Historical reform and adoption of calendars have changed datekeeping practices. The introduction of the Gregorian reform in 1582, for example, altered date sequences in many regions, and different areas adopted it at different times. Accurate datekeeping underpins chronology, legal records, science, and data management, making precise dating essential across disciplines.