Date validation for booking system

validates_date

...

  field :started_at, type: Date
  validates_presence_of :started_at, message: 'you have to provide a start-date'
  validates_date :started_at, on_or_after: :today, on: :create
  validates_date :started_at, is_at: lambda { |_self| _self.started_at_was }, on: :update, if: lambda { |_self| _self.started_at_was.to_date < Time.now.to_date }
  validates_date :started_at, on_or_after: :today, on: :update, if: lambda { |_self| _self.started_at_was.to_date >= Time.now.to_date }
  
  field :ended_at, type: Date
  validates_presence_of :ended_at, message: 'you have to provide an end-date'
  validates_date :ended_at, on_or_after: lambda { |_self| _self.started_at }
  
...