chronological_age.rb

  # Return as a three-element array the number of years, months, and days
  # between the two given dates.
  #
  # Time is counted from the "from" date, in the order of years,
  # months, and days. This order is significant since leap years will
  # pass unnoticed if they are "absorbed" by a year or month
  # calculation.  For example, 2007 to 2008 is a year and February 15th
  # to March 15th is a month, whether they include a leap day or not.
  # However, the leap year is observed properly if it occurs within the
  # days portion of the calculation. For example February 20th to March
  # 3rd may be 12 or 11 days, depending on whether it's a leap year.
  def age(from_date, to_date)
    return nil if from_date.nil? || to_date.nil?
    from, to = from_date, to_date.dup
    
    age_years = to.year - from.year

    # Carry a year?
    if from.month > to.month
      age_years -= 1
      carry_months = 12
    else
      carry_months = 0
    end
    
    age_months = to.month + carry_months - from.month
      
    # Carry a month?
    if from.day > to.day
      age_months -= 1
      if age_months < 0
        # carry again
        age_years -= 1
        age_months += 12
      end
      # Carry the number of days in the month before the "to" month
      carry_days = Time.days_in_month(to.last_month.month, to.last_month.year)
    else
      carry_days = 0
    end
    
    age_days = to.day + carry_days - from.day
      
    [age_years, age_months, age_days]
  end
  
  # Return the chronological age of the subject at the time of the test's
  # administration as a 2-element array of years and months.
  #
  # If the day component of the subject's age is more than 15 days, then the
  # month is rounded upwards.
  def chronological_age
    return nil if subject.nil? || subject.date_of_birth.nil? || date_of_administration.nil?    
    age_years, age_months, age_days = age(subject.date_of_birth, date_of_administration)
        
    # Round up if they're more than halfway through the month.
    if age_days > 15
      if age_months == 11
        age_years += 1
        age_months = 0
      else
        age_months += 1
      end
    end
    
    [age_years, age_months]
  end
  
  # Like #chronological_age, but return the result as an integer of total
  # months.
  def chronological_age_in_months
    years, months = chronological_age
    if years.nil? || months.nil?
      nil
    else
      (years * 12) + months
    end
  end