Endless space and time
Sailor Pluto (guardian)
The door was decorated with the phases of the moon in the anime. It first appeared in episode 82 when Sailor Pluto opened the Door to allow Sailor Moon, Tuxedo Mask, the Inner Senshi, and Chibiusa to pass through.
It was shown only in the second saga of the second season when Sailor Moon, Tuxedo Mask, and the four Inner Guardians came with Chibiusa to reach the thirtieth century. Sailor Pluto suddenly appeared in front of the Door, intending to strike any trespassers with her Garnet Rod. After understanding their mission, she opened the Space-Time Door in order for them to go to the future, Crystal Tokyo.
Sailor Moon Crystal
Just like the manga, the Space-Time Door was a similar role between space and time itself.