Historical Dates with wp 2.6

I have used this way around to allow dates before 1969 on my WordPress. Since I upgraded to 2.6 it doesn’t work anymore. I tried to make changes again but it looks like the file ‘functions.php’ in the ‘wp-includes’ directory that I need to modify has been completely been rewritten.
Can someone post here how to make the changes now? It would be very useful to those of us who want to post about the past. Thanks.