The original Japanese is "Toki no Yami", though, isn't it? That apparently translates to "Darkness of Time", according to the translators. (Or, more accurately, "Time's Darkness", since Japanese doesn't have the concept of "of"...)
So Darkness Beyond Time is a mistranslation. (Not as terrible a one as Time's Eclipse, though...)
Darkness Beyond Time sounds so much cooler, though, in my opinion. :p
And a slogan...I can't think of any. :(
Edit: Got one:
Chrono Compendium - The world exists in us.
(Modified quote from NPC in Enhasa)
Most of the good ones were already taken, so sorry if it sucks. :/