This came up on the mailing list.
yyyy-mm-dd (ISO 8601) is the internationally accepted format for numeric
dates. This commit changes all BIPs to use that instead of dd-mm-yyyy.
It also updates BIP 0001 to prescribe the new format.
Per the linked commit, this rule became active for blocks with timestamps after on March 15, 2012 at 00:00 UTC (February 20, 2012 at 00:00 UTC for testnet).