Via:
For the Americas, the expansion will be available to play at 9:00 p.m. PDT on March 24.
10:00 p.m. MDT on March 24
11:00 p.m. CDT on March 24
12:00 a.m. EDT on March 25
1:00 a.m. ADT on March 25
1:30 a.m. NDT on March 25
12:00 p.m. SGT on March 25
3:00 p.m. AEDT on March 25
If you want to burst into the new levels immediately, be logged in, playing, and be ready to kill Diablo several minutes after your time zone’s release. Then, you can move right into it – won’t even have to restart the game client (they claim).