class Date

def as_json(*)


# => #
Date.json_create(x)

\Method +JSON.create+ deserializes such a hash, returning a \Date object:

# => {"json_class"=>"Date", "y"=>2023, "m"=>11, "d"=>21, "sg"=>2299161.0}
x = Date.today.as_json
require 'json/add/date'

returning a 2-element hash representing +self+:
\Method Date#as_json serializes +self+,

see Marshal[https://docs.ruby-lang.org/en/master/Marshal.html].
to serialize and deserialize a \Date object;
Methods Date#as_json and +Date.json_create+ may be used 
def as_json(*)
  {
    JSON.create_id => self.class.name,
    'y' => year,
    'm' => month,
    'd' => day,
    'sg' => start,
  }
end