I have a program that outputs bugs fixed on given dates. I am going to graph this in excel, but I want to center the graph at the release date so I can see the change of bugs fixed before release and after. To do this, I need the x-axis to be the difference from the date...

therefore, if the release date is n... the x-axis would be like:
... , n-3 , n-2 , n-1 , n , n+1 , n+2 , n+3 , ...
instead of chronological dates.

My dates right now are stored in a hash, and i output them (with their associated bug ticket tally count) like this

open (OUTPUT, ">output7.3.0.txt") or die $!;
      foreach my $date (sort  {$a <=> $b} keys %dates) {
         print OUTPUT "$dates{$date}{date}\t$dates{$date}{count}\n";
close OUTPUT;

My dates in the hash are stored in this format: 21-Jun-07

If the release date is a fixed constant, how would I be able to find the difference between the release date and the different dates in the hash table (whether it be n+x some number or n-x) so that I would be able to graph the dates.