[Bug libstdc++/114240] sys_days not being parsed with only a date in the stream

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114240

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This has revealed another bug in some of the from_stream overloads, due to
Clock::from_sys / Clock::from_utc sometimes returning a higher precision value
than the input argument (due to using the common_type with seconds).