It is 10pm EST—Hortus just posted the following:
2.4 PTR Questions and Announcements
There are no pre-made characters for this test phase.
Authentication and eligibility
The PTR authentication server was updated on February 1st 2008, if your account was not active on this date you will not have access to the 2.4 PTR.
Battlegrounds and Arenas
Because 2.4 includes major changes to the Warsong Gulch battleground and we would like to get extensive testing there all other battlegrounds and arenas have been temporarily turned off.