I noticed that too. If I do
gmt math -Q -fT 2021-10-05T05:05:05.00001 1 ADD --TIME_UNIT=d =
then it rounds up to 5, so it is some sort of roundoff since internally every time is a double precision days since the epoch which is at 1970. So undoing the formatting and just getting the days
gmt math -Q 2021-10-05T05:05:05 1 ADD --TIME_UNIT=d =
18906.2118634
gmt math -Q 2021-10-05T05:05:05 0 ADD --TIME_UNIT=d =
18905.2118634
they are 1 apart, but clearly breaking this down to hh:mm:ss suffers from roundoff…
Will have to debug at some point.