A v2 of this series has been posted with a revised approach based on reviewer feedback. The v2 replaces format_to/ostreambuf_iterator with a stack-buffer + __ostream_insert approach that correctly handles the stream sentry, width/fill alignment, failbit semantics, and exception safety.
The _Streambuf_sink optimization has been split out from this series, per Tomasz's suggestion. It will be submitted as a separate follow-up patch, where it can also benefit C++23 std::print and user-written format_to(ostreambuf_iterator, ...) independently of the chrono changes. See the new thread: [PATCH v2 0/2] libstdc++: Optimize chrono ostream insertion via __chrono_write
