Explore fast calendar algorithms implemented in the Linux Kernel and libstdc++ in this conference talk from CppNow 2023. Dive into optimization techniques that outperform counterparts in popular libraries, including strength reduction methods and improved instruction level parallelism. Learn about a highly efficient implementation of std::chrono::year::is_leap, a concise expression for determining the last day of a month, and the historical context of our modern calendar. Gain insights into how decisions made centuries ago impact the performance of today's calendar algorithms, presented by Cassio Neri, a PhD holder in Applied Mathematics with extensive C++ experience in the financial industry.