Of course time travel in a forward direction is well proven but the effect of gravitation still remains a kind of mystery (to me anyhow)
One Mark in another thread speaks of time travel with reference to the center of an explosion, intriguing.
http://able2know.org/topic/217655-2#bottom
Of course at that instant the concentration of matter might engender a local increase in gravitation which would indeed slow any clock capable of withstanding it but wouldn't the diff be very slight
….suggesting another q: If gravitation slows a clock, might a high pressure not also
....and how about the effect of acceleration, terrific I'd think near the very center