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.