date-quarter.rb

class Date
  def quarter
    case self.month
    when 1,2,3
      return 1
    when 4,5,6
      return 2
    when 7,8,9
      return 3
    when 10,11,12
      return 4
    end
  end
end

# Usage example:

Date.parse("1 jan").quarter # -> 1
Date.parse("1 apr").quarter # -> 2
Date.parse("1 jul").quarter # -> 3
Date.parse("1 oct").quarter # -> 4
Date.parse("31 dec").quarter # -> 4