The abandonment hatch to Test Shaft 09 Zulu Bunsen is dated 15 June 1961. Beyond that, the area with the foyer, the elevator tower, and Pump Station Alpha is dated 1952. Test spheres 1, 2, and 3 are dated 1953, 1957, and 1958, respectively. Then you get to Pump Station Beta... which is dated 1971, a decade after the shaft was supposedly abandoned. The fourth and fifth spheres are dated 1972 and 1976. The two office areas next to Pump Station Gamma are dated 1978 and 1981, while the pump station itself is dated 1982. The sixth and final test sphere has two different dates on different parts of the wall surrounding the conversion gel pipe, 1982 and 1986.

Of lesser annoyance is the fact that we only see six spheres, despite the diagram in the first sphere clearly showing nine.

Just further proof that Aperture Science was batshit crazy.

Or the automated systems put them in place, and dated them properly.

And here i thought the biggest plot hole was the lack of cake.