damn-you-tim.rb

LETTERS = "abcdefghijklmnopqrstuvwxyz".split('')

def base_alpha_26(int)
  num = int
  letters = []

  i = 0
  i += 1 until 26**i > int

  while (i -= 1) >= 0;
    letters << (num / 26**i)
    num = num % (26**i)
  end

  letters.map { |l| LETTERS[l-1] }.join
end

puts base_alpha_26(Integer(ARGV[0]))

gistfile1.txt

 $ ruby damn-you-tim.rb 1000000000
cfdgsxl

Just for good measure, golfed.

def base_alpha_26(i)
  l,n,e,j=('a'..'z').to_a,i,[],0;j+=1 until 26**j>i;while(j-=1)>=0;e<<n/26**j;n%=26**j;end;e.map{|k|l[k-1]}*''
end