test.rb

require 'active_record'
require "active_support"

class Person < ActiveRecord::Base
  establish_connection adapter: 'sqlite3', database: 'foobar.db'
  connection.create_table table_name, force: true do |t|
    t.date :date
  end

  validate :is_date?

  def is_date?
    errors.add(:date, "must be a valid date") unless date.try(:to_date)
  end

end

p1 = Person.new(date: Date.current)
p p1.errors unless p p1.valid?

p1= Person.new(date: 'wrong')
p p1.errors unless p p1.valid?


#> ruby test.rb
#true
#false
#<ActiveModel::Errors:0xb9486598 @base=#<Person id: nil, date: nil>, @messages={:date=>["must be a valid date"]}>
#<ActiveRecord::Relation []>