SE has used the 24h clock as long as I remember from ffxi and continuing to ffxiv.


Edit:
Actually, here is the schedule for the 10/1 maintenance, from the lodestone:

[Date & Time]
Oct. 1, 2013 from 12:00 a.m. to 2:00 a.m. (PDT)
* The completion time may be subject to change.

Pretty cut and dry, I think.