The Restroom on The Edge of Forever

Technically, it's restrooms, since it affects both a ladies and a gents, but still the name is kept singular.

As you've no doubt guessed, those in the know can enter these restrooms and, by saying or doing the right thing, open a portal in time and space (well, space being that of the surface of Earth). Generally speaking, the trip into the past lasts between 2 and 7 days.

Visually speaking, the restrooms look like medium sized examples of their kind. They are always quite clean and the soap in the dispensers always smells of lemons.