The SRFI defines:

"The nanoseconds component — The partial second added to seconds.
Always non-negative and less than 10^9. If seconds is non-negative,
this is the number of additional nanoseconds to go toward the future
after seconds. If seconds is negative, this is the number of
additional nanoseconds to go further back in time before seconds."

Then how can we represent the time "0.5 seconds before the epoch"?

As far as I know about POSIX (i.e. the bare minimum to survive),
timespecs are not supposed to be able to represent time before the
epoch, hence the incompleteness.