A curcial step in the conversion, namly the computation of the actual time from
the seconds since the epoch, had been conducted with too little precision. Now
the precision is fixed to a high value throughout the whole computation, and the
tests succeed again.
This test could not have been added before, because conversion was not
guaranteed to work for dates before the unix epoch. It now is, and we finally
can check the start of the NTP epoch! :)