Mars
Mars has existed for approximately 4.6 billion years, forming from the same protoplanetary disc that birthed our solar system. Its longevity is essentially assured for billions of years hence, barring catastrophic collision or the eventual expansion of our Sun into a red giant phase.
The planet has witnessed epochs of which humanity can scarcely conceive. It bore liquid water when Earth was still cooling, lost its magnetic field when our ancestors were single-celled organisms, and will continue its silent orbit long after the last Monday has been observed by any sentient being.
Monday
Monday, as a human construct, possesses a considerably shorter tenure. The seven-day week emerged approximately 4,000 years ago in Mesopotamia, though the specific designation of Monday required subsequent millennia of cultural development. Its existence is entirely contingent upon the continuation of human civilisation and its timekeeping conventions.
However, within the framework of experiential longevity, Monday demonstrates remarkable persistence. Each individual Monday may last only 24 hours, yet the institution regenerates with absolute reliability. Humanity has endured an estimated 104,000 Mondays since the adoption of the seven-day week, each one indistinguishable from its predecessors in its fundamental nature.