Gates (Useful Life of)
Several Gates exist in the world to facilitate Crono's quest in defeating Lavos. But do the Gates terminate after a certain period? While the nature of the Gates dictate that they always project the user the same period of time forward or backward in history, do the Gates themselves exist for all time?
Leebot, The Unknowuser_
The Gates have a set lifetime defined by Time Error (that is, from the perspective of the End of Time). For instance, to avoid any temporal mishaps throughout history, such as other people finding the Gates and using them, the Entity anticipated that Crono and his group would only take a few weeks or months to defeat Lavos, and accordingly phased the Gates out at a certain time after its expectation for the completion of the quest. Once Crono actually finished, the Entity closed the Gates arbitrarily, as they were then definitively no longer needed.
The main problem here is that the Gates don't exist in other eras where spatially they should sit. However, each Gate is also presumably located in an inaccessible or nonexistent spot of land throughout the eras. To explain, take the Gate in Guardia Forest, 1000 A.D. In 65000000 B.C., that Gate might exist, but it may not be found anywhere or may simply hover above the ocean. It could also exist within a cliff face or lava flow; the party would never see it. The Gate at the Bangor Dome may sit several feet above the ground level of 1000 A.D. and other eras, due to sedimentary build-up. This can be applied to every Gate except interestingly the Truce Canyon / Leene Square Gate, which appear to be in the same area and can be interpreted as one Gate (with simply two openings on the other side of 400 years). With geographical inaccessibility considered, the Gates might actually exist in every era.
The only problem is, who determines what end of the Gate potential entrants are in when they access a Gate? What would determine whether someone entering the Tyrano Lair Gate in 30000000 B.C. went 65000000 B.C. into the past or the future? This is problematic, as if anyone entered the Gate in a strange era, he or she could potentially disrupt history on a huge scale.