The (regular) Ecliptic Calendar can be adopted by anyone at any time, without regard to whether anyone else adopts it too. One marks time for oneself entirely by celestial motions, and in perfect accord with the astronomical seasons.
 
The Perennial Ecliptic Calendar is similarly perfectly aligned with the astronomical seasons, but it also specifies days of work, market days, days of rest, and holidays in such manner that each month has exactly four weeks, though the weeks vary between seven and eight days. Since one works (and rests) relative to a larger society, it cannot be adopted by individuals, but only by whole nations of people.
 
A full description of the Perennial Ecliptic Calendar is given, in English, through the link below.