Epoch (Transferal to 2300 A.D.)
Dalton states that he recognizes the Epoch as something Belthasar was working on in Zeal before being banished. How did the Epoch come to the future? It is not depicted as traveling with Belthasar to 2300 A.D. after the Ocean Palace Incident.
The Epoch may have been built up enough in Zeal to support time traveling capacity. Under this idea, the Epoch would have traveled to find Belthasar in the future shortly after he went there following the Ocean Palace Incident. There, Belthasar probably only had to finish helm controls or other items to ready the craft for Crono.
Belthasar simply copied the design of the Epoch in 2300 A.D., able to construct it much more rapidly there thanks to the future technology and his skill in magic.
De Lorean Storage
The Epoch prototype survived the Fall of Zeal and merely sat unused for thousands of years until Belthasar excavated it in the future.
From: Theory (Objects)