Mimesis-republic cascading.jruby

How can we easily calculate the age from a birthdate.

I was thinking of :

def compute_age_birthdate
  dateCurrent = Time.now.strftime("%m-%d")
  yearCurrent = Time.now.strftime("%Y")
  parse_date birthDate, :pattern => "yyyy", :into => "birth_year"
  parse_date birthDate, :pattern => "MM-dd", :into => "birth_date"
  age = yearCurrent - "birth_year"

  compute_age = """ birth_date > dateCurrent  ? age : age - 1  """
  eval_expression compute_age, :into => "age"
end

is there a better way ? it also raise the question : Howto simply create calculated Fields? here we have age - 1 for example. bu i will also have a need like ROUND(totalPlayTime / 1000, 1 )