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The most common type of pre-modern calendar was the lunisolar calendar, a lunar calendar that occasionally adds one intercalary month to remain synchronised with the solar year over the long term.
This is done by giving names to periods of time, typically days, weeks, months and years.
Similarly, Michael Rappenglueck believes that marks on a 15,000-year-old cave painting represent a lunar calendar.
Nearly all calendar systems group consecutive days into "months" and also into "years".
This resulted in an observation-based lunar calendar that shifts relative to the seasons of the solar year.
The first calendar reform of the early modern era was the Gregorian calendar, introduced in 1582 based on the observation of a long-term shift between the Julian calendar and the solar year.