Zero Time is basically a space that existed outside of time in its own space, and it's the only place that Cybodies could function.

[ Trying to ignore the salute. Reminds him a little too much of Kiraboshi. ]

Cybodies are... Well, were mechs from another planet that might have been people at one point, and were largely sealed in Zero Time because of Samek, the Emperor Cybody, who was going to drain the planet of libido if Zero Time was destroyed and he was set free.