Hi, the situation is likely the one described in another thread (I am not allowed to add a hyperlink, because I am a new user)
Of course, I have also already tried the solution suggested in that thread (i.e. changing the svn:date property in my case from 2003-01-01 to 2009-05-16), and checked that it really has changed. (What I've done for checking was: opened the property dialog with TortoiseSVN, issued the svnlook date command, and even tried svn log -r {2009-05-01}:{2009-05-25}
According to all three checks, the timestamp of the last revision was set correctly (i. e. the first two checks returned the timestamp I had just set, and the log of the head revision was dumped as expected for the specified time interval.)
Thus the svn:date property seems to be set correctly, however, when I tell TortoiseSVN to display the list of logs, it still displays the entry for the head revision as if it had been committed with the old timestamp (2003-01-01). I have already updated the working copy, and even checked it out again, and before that I manually updated the timestamp of the file corresponding to the head revision, but still the old date is displayed by TortoiseSVN.
After struggling with this issue for hours, I really do not know where else can that timestamp be stored. (And I am almost convinced that this whole issue is not a TortoiseSVN bug, as checking it out again should have removed the problem in that case)