When I write dates in file names, I use the format YYMMDD. I picked this style up from Yosufi, who used it to name files when Windows was the popular operating system, and you had only eight + three letters for file names. He would use two letters and the date, for the eight letters, and three letters for the extension. The two letters were meaningful to him, usually about the contents of the files.
I continued the numbering scheme because the date lists correctly when sorted in a directory listing. If I named the files like everyone else, they wouldn't list correctly. As an example, how about a database dump from this month, last month and the month before. If I used the format that most people seem to use, they would list like this:
dump_011009.sql dump_020809.sql dump_122208.sql
That is clearly not in the order I want to see them, which would be in creation order. Worse, what if I had dumps from this time last year, too?
dump_011009.sql dump_011108.sql dump_020809.sql dump_042908.sql dump_122208.sql
Now, which files, at a glance, are the most current? I can't tell. I can't tell at a glance if the database dump for this month happened. Better to use the year first, followed by month, then day. All of the files list correctly:
dump_080111.sql dump_080429.sql dump_081222.sql dump_090110.sql dump_090208.sql
So, next time you decide to write a date in a file name use the year, month THEN day in the filename. Trust me on this.