The biggest challenge facing primitive calendars everywhere is the fact that the two most obvious, natural systems of timekeeping are incompatible: that is to say, twelve lunar months, from new moon to new moon, add up to just over 354 days; and this cannot be made to match in any convenient way the 365¼ days of the solar year, which is the time it takes for the earth to make one complete circuit of the sun, from spring equinox to spring equinox, for instance.
