Tests are not timed on OS X

[swaroopbekal]

The BSD date utility in OS X doesn't recognize %N as a valid format specifier.

[rpavlik]

I've partially worked around this in my modified version at https://github.com/rpavlik/rpisudiss/blob/master/tests/sh2ju.sh

[swaroopbekal]

That's good. You can also check if uname -s returns "Darwin" if you want to make the expression simpler.

[rpavlik]

I'd rather leave it as a functional test, rather than an identity test, personally, since it checks exactly what it needs (so would work elsewhere with either date utility)