DouKaku?

#!/usr/bin/env ruby
require 'date'

from = DateTime.now
to = DateTime.parse("2013-12-31")

friday = (from..to).inject(0) do |friday, date|
    if date.mday == 13 and date.wday == 5 then
        puts date.strftime('%Y-%m-%d')
        friday + 1
    else
        friday
    end
end

puts friday

Sub Main()
  t = Date
  p = 0
  Do
    If DateValue("2013/12/31") = t Then
      Exit Do
    End If
    If Day(t) = 13 And Weekday(t) = 6 Then
      Range("A1").Offset(p) = t
      p = p + 1
    End If
    t = t + 1
  Loop
End Sub