No, because things in time don't happen until they are actually done.
what?
I think what he's saying is this:
Crono and Co. go back in time a slightly alter events in 12,000 BC. However, FIRST, 12,000 BC passes "naturally" so to speak. THEN 1000 AD rolls around, Crono starts his adventuring, and then goes back and changes 12,000 BC. Makes my head hurt kind of, because after this, the old timelone is eliminated meaning things ALWAYS happened as Crono had affected them, but then by law of the Time Bastard Theorem, Magus and the Gurus still remember the old timeline that never was, etc...
Ugghhhh, time travel causes insanity I'm sure of it!