First, most filesystems don't handle well too much files in a directory. Some do like reiserfs on linux.

If you have control on the adding process in your directory, you can name the file so that the first in lexicographically order is the oldest. With a smart filesystem like reiserfs the time spent to find a file.will be O(log(n)) and not O(n).

I stress it again: if you don't have a smart file system, it is madness to shove 10000 files in a directory.

With a dumb filesystem, you must do what the filesystem should have transparently done for you (by implementing internally a directory as a tree). . For example you create a hierarchy and you create the file in the right place: if it arrives at 3:04 you create the file in /var/spool/whatever/03/04. Then finding the oldest file is just done by walking the tree.

-- stefp