validates :z, :presence => true, :numericality => {:greater_than => 0}
validates :t, :presence => true, :numericality => {:greater_than_or_equal_to => 0}
validates :z, :presence => true, :numericality => {:greater_than => 0}
validates :t, :presence => true, :numericality => {:greater_than_or_equal_to => 0}