I’ve been a fan of the ISO 8601 date format ever since I first learned about it at a Dublin Core Metadata Initiative conference in the late nineties: It’s:

• Logical (the elements are ordered from the most significant to the least from left to right);
• Y2K-proof, since it uses a 4-digit year (remember this was a big issue in the late nineties!);
• Internationally unambiguous (unlike the absurd U.S. MM/DD/YYYY format);
• Extensible to include international time to any level of precision
• Recommended by both the DCMI, the W3C, AND the web comic XKCD!

XKCD Public Service Announcement: ISO 8601

Plus if you use it for file names, they even sort in the correct order.

You should use it, too!