My ISO-8601 software conversion includes fractions of days because I
believe sometime in the future it will be included.

ISO-8601 is unusual in that it has fraction of minutes and
hours. Fraction of days is a logical extension.

The question, is fractions of days time or not?

If it is time, than the format should be
YYYY-MM-DD**T**,5

If not time, than the format should be
YYYY-MM-DD,5 or .5

My argument is that all fractions are misrepresented.

YYYY-MM-DDThh:mm:ss,ssss is a
misrepresentation for this reason.

SS is base 60 while the ,sss is base 10.
The implication of ,sss is that it is the same base as the SS seconds. So
also hh,hhh that .hhh is base 24.

NASA and Microsoft have fractions of time as .fff base
10.

One way to rationalize time is that time has elements that
are base-24 and base-60

HH = base 24

MM= base 60

SS =base 60

Therefore fraction of days is not time. Therefore the format should
be:

YYYY-MM-DD.ffff, much more logical to humans.

Parsing the format is also many time easier.

hjw