The Long Now Guiding Principles

Longevity: The clock should be accurate even after 10,000 years, and must not contain valuable parts (such as jewels, expensive metals, or special alloys) that might be looted.
Maintainability: Future generations should be able to keep the clock working, if necessary, with nothing more advanced than Bronze Age tools and materials.
Transparency: The clock should be understandable without stopping or disassembling it; no functionality should be opaque.
Evolvability: It should be possible to improve the clock over time.
Scalability: It should be possible to build working models of the clock from table-top to monumental size using the same design.