section_path: Systems
section_name: Systems
paragraph_text: Systems
  
  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.
  
  The simplest calendar system just counts time periods from a reference date. This applies for the Julian day or Unix Time. Virtually the only possible variation is using a different reference date, in particular, one less distant in the past to make the numbers smaller. Computations in these systems are just a matter of addition and subtraction.
  
  Other calendars have one (or multiple) larger units of time.
  
  Calendars that contain one level of cycles:
  
  Calendars with two levels of cycles:
  
  Cycles can be synchronized with periodic phenomena:
  
  Very commonly a calendar includes more than one type of cycle or has both cyclic and non-cyclic elements.
  
  Most calendars incorporate more complex cycles. For example, the vast majority of them track years, months, weeks and days. The seven-day week is practically universal, though its use varies. It has run uninterrupted for millennia.