The Royal Observatory
The Royal Observatory in Greenwich, outside London, England
This is where they keep the Prime Meridian—well, the marker for it—the line that marks 0° longitude and divides the Earth into East and West hemispheres. Inside are loads of historic scientific devices, including the four clocks made famous by the book Longitude.
A digital clock outside, set right at the Prime Meridian, lets you know down the millisecond exactly what time it is — the original Greenwich Mean Time, by which most of the world sets its clocks.