On Tue, 26 Feb 2013 17:57:22 -0800, "joeu2004" wrote:

But it does not say __which__ Aug 1st. I can imagine that the date of birth
is the "closest" Aug 1st, with a rule to handle exactly between two firsts
of Aug in leap years.

For the American Jockey Club -- http://www.jockeyclub.com/pdfs/rules_12_final.pdf

Age of the Thoroughbred: For the purpose of determining
age, the date of birth for all Thoroughbreds is deemed
to be January 1 of the year of foaling.

So, at least in the US, a horse born on Dec 31, 2011 would have been "born" on Jan 1, 2011.

Hmmm, wonder if there's a similar set of rules for the Australian Jockey club: