Presence true using validate_timeliness gem

Question

Is there any built in method required using validate_timeliness? I want my birthday field to be required, when I separated the presence(like this)

Model

# validates :birthday, 
    #   :presence => true,
    #   :on => :create

it will show both errors(first error will be birthday can't be blank and the other one is birthday is not a valid date).

Model

validates_date :birthday, :if :birthday,
                              :before => lambda { 18.years.ago },
                              :before_message => "must be at least 18 years old"
                              :end

Question: How can I check if my birthday is not a nil?

Note: I also tried this

Model

  validates_date :birthday, :if birthday.present?,  # here's the line giving an error
                              :before => lambda { 18.years.ago },
                              :before_message => "must be at least 18 years old",
                              :end

but it gives me an error expecting keyword_end