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".
Such ideas are mooted from time to time but have failed to gain traction because of the loss of continuity, massive upheaval in implementation, and religious objections.The seven-day week is practically universal, though its use varies. The length of the interval between two such successive events may be allowed to vary slightly during the year, or it may be averaged into a mean solar day. Because the length of the lunar month is not an even fraction of the length of the tropical year, a purely lunar calendar quickly drifts against the seasons, which do not vary much near the equator.It does, however, stay constant with respect to other phenomena, notably tides. Alexander Marshack, in a controversial reading, believed that marks on a bone baton (c. Other marked bones may also represent lunar calendars.A full calendar system has a different calendar date for every day.Thus the week cycle is by itself not a full calendar system; neither is a system to name the days within a year without a system for identifying the years.