https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110504
--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> --- I have a patch to fix this: --- a/libstdc++-v3/include/bits/chrono_io.h +++ b/libstdc++-v3/include/bits/chrono_io.h @@ -1079,7 +1079,9 @@ namespace __format { locale __loc = _M_locale(__ctx); auto __ss = __hms.subseconds(); - if constexpr (is_floating_point_v<rep>) + if (__ss == __ss.zero()) + ; // Do not write all-zero fractional part. + else if constexpr (is_floating_point_v<rep>) { __out = std::format_to(__loc, std::move(__out), _GLIBCXX_WIDEN("{:.{}Lg}"), However the example in [time.clock.utc.nonmembers] suggests that in fact printing the zeros is correct. The sys_time<milliseconds> value is printed as "2015-06-30 23:59:60.000 UTC" which would be altered by the patch above.