gistfile1.rb

#!/usr/bin/env ruby

class Dre
  MONTHS = %w(january february march april may june july
            august september october november december)

  def self.parse(string, options = {})
    new(options).parse(string)
  end

  def initialize(options)
    @now = Time.now
    @options = options
  end

  def parse(string)
    string.downcase!

    normalize_months(string)

    day = extract_day(string) || @now.day
    month = extract_month_index(string) || @now.month
    year = extract_year(string) || @now.year

    Time.local(year, month, day)
  end

  private

  def normalize_months(string)
    {
      /jan/ => 'january',
      /feb/ => 'february',
      /mar/ => 'march',
      /apr/ => 'april',
      /jun/ => 'june',
      /jul/ => 'jul',
      /aug/ => 'august',
      /sep/ => 'september',
      /oct/ => 'october',
      /nov/ => 'november',
      /dec/ => 'december'
    }.each do |match, replacement|
      string.sub!(match, replacement)
    end
  end

  def extract_day(string)
    string =~ /\b(\d)(?:th|rd|nd|st)?\b/ && $1.to_i
  end

  def extract_month_index(string)
    MONTHS.each_with_index do |month, index|
      return index + 1 if string.include?(month)
    end
  end

  def extract_year(string)
    string =~ /(\d{4})/ && $1.to_i
  end

end

p Dre.parse("3rd of Jan 1998") #=> 1998-01-03 00:00:00 +0000
p Dre.parse("feb 5th 2001")    #=> 2001-02-05 00:00:00 +0000
p Dre.parse("August 4")        #=> 2012-08-04 00:00:00 +0100