Norbert Wójtowicz's wroclove.rb 2025 single-speaker talk titled 'Practice What You Preach: Time, the Gregorian Calendar'. Uses the 13,000-year history of human calendars as a jumping-off point for three engineering questions: how to make computers go fast, how to deploy breaking changes, and when to rewrite. Historical arc: lunar vs solar calendars, Sumerian base-60 math, Egyptian 365-day calendar with 5-day New Year's Eve party, Romans losing prior knowledge (Silicon Valley-tech-bro analogy, library of Alexandria destroyed by Roman neglect), 10-month Roman calendar with an un-counted winter, Julian reform 45 BC, Augustan fix every 4 years, Pope Gregory's 1582 reform with the 4/100/400 rule. Performance section: leap-year and days-in-month benchmarks showing branch-free / lookup-free code can be 3–4× faster on modern CPUs due to branch prediction and memory latency; .NET 7 adopted the Neri–Schneider algorithm (2-year-old paper) giving 4× speedup over .NET 6 for epoch-to-date conversion. Teaches rephrasing problems with Zeller's computational calendar (start year in March), convenient number tricks (n/5 ≈ 2n/10), and Euclidean a-functions. Breaking-changes section: the computus problem (origin of the word 'computer'), Bede's 725 hand-computation of 532 years of Easter dates, Gregory's techniques for a 1600-year-old customer — simple patch instructions on top of the old rule, fixing the drift while at it, one Thursday-to-Friday switch, 120-year adoption window. The Wikipedia list of Gregorian-adoption dates shows Saudi Arabia in 2016, France in 1805, yellow-highlighted countries that flipped back and forth, and German/Swiss cities adopting 200+ years apart. Rewrites section: Norbert's golden rule — rewrite if and only if you build on different principles (axioms, not platitudes like clean code or TDD). Integer-seconds vs milliseconds thought experiment about software going into a Boeing. C++ Chrono library by Howard Hinnant presented as the exemplar: durations as representation+ratio, time points as duration+clock, calendars as a separate abstraction, no auto-conversion of '31 January + 1 month', compile-time type-safe arithmetic with no runtime cost, four C++ committee passes to standardize. Closing: always measure, study the past, cross-pollinate ideas between language communities (port Chrono to Ruby), don't vibe-code critical thinking.