At present ntptime doesn't care about sub-second part (fraction part of NTP reply.)

I've wrote some lines which handles it and syncs RTC milliseconds-accurate.
https://github.com/puhitaku/micropython/commit/5dd5bed21677250f1b26da2a48558b03c08d9ca1

I think it won't make any incompatibility even if I change this API to set millis implicitly because utime.datetime doesn't care about millis but I can't guarantee it so it's why I adopted kwarg.

Takumi