class DateTime
  def httpdate
    t = dup.utc
    sprintf('%s, %02d %s %d %02d:%02d:%02d GMT',
      Time::RFC2822_DAY_NAME[t.wday],
      t.day, Time::RFC2822_MONTH_NAME[t.mon-1], t.year,
      t.hour, t.min, t.sec)
  end

  def utc
    to_s =~ /([-+])(\d\d:\d\d)$/
    sign, hours, mins = $~.to_a[1..3]
    diff = hours.to_i * 60 + mins.to_i
    signed_diff = sign == "-" ? -diff : diff
    self - (signed_diff / 1440.0)
  end
end