validates is_good

model.rb

validate :is_valid_time

def valid_time?
  self.time > 24.hours.from_now
end

def is_valid_time
  errors.add("Your object", "is not valid. Other explainations.") unless valid_time?
end

validates..rb

class Post < ActiveRecord::Base
  ...

  validate :date_scope

private
  def date_scope
    if Post.where("user_id = ? AND DATE(created_at) = DATE(?)", self.user_id, Time.now).all.any?
      errors.add(:user_id, "Can only post once a day")
    end
  end
end