If you want to flatten the directory structure (thus sorting by date over all files in all directories, ignoring what directory the files are in) the find-approach suggested by @yeti is the way to go. If you want to preserve directory structure, you might try
$ ls -ltR /path/to/directory which sorts directory based.
Alternatively, if you want a recursive (-R) listing, and you want the files listed in reverse (-r) order (i.e. from oldest to newest):
$ ls -ltrR /path/to/directory 
