The implementation of Unix.stat
can be seen at the Github mirror of Inria's codebase. As you can see the portable second-precision time values are converted directly into doubles, ignoring the newer nanosecond precision values available in the struct stat
.
Please consider submitting a bug report so that this can be improved.
In the meantime if you desperately need this functionality and simply can't wait, you could write your own improved stat_ext
that returns sub-second time stamps. Of course it is best to avoid reimplementing the stdlib unless there is great need.