https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114244
Bug ID: 114244 Summary: Need to use round when parsing fractional seconds Product: gcc Version: 14.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: libstdc++ Assignee: unassigned at gcc dot gnu.org Reporter: howard.hinnant at gmail dot com Target Milestone: --- I'm not sure if this is a bug or just QOI. But consider: #include <chrono> #include <string> #include <iostream> using time_point_t = std::chrono::sys_time<std::chrono::milliseconds>; time_point_t decrypt(std::string s) { time_point_t tp; std::istringstream in(s); in >> parse("%Y%m%d-%T", tp); return tp; } int main() { std::cout << decrypt("20240304-13:00:00.002"); } I expect the output to be: 2024-03-04 13:00:00.002 but it is: 2024-03-04 13:00:00.001 I believe the issue is the use of duration_cast<milliseconds> or time_point_cast<milliseconds> somewhere under the parse (not sure exactly where). Since 0.002 is not exactly representable in floating point, we're getting a truncate towards zero because this parses as just barely under 0.002. I highly recommend the use of round<Duration> when converting floating point based durations and time_points to integral based.