v.0.2 has both conversion methods in TimeUnit. The unexpected weirdness is that convert(Duration) saturates while toDuration throws ArithmeticException, but both seem author-culture-consistent. Perhaps TimeUnit#toDuration doesn't provide enough value in view of the existing Duration.of and TimeUnit#toChronoUnit. And most of the time you'd expect to convert from Duration to long, just before calling a TimeUnit based method.
/** * Converts the given time duration to this unit. * * @param duration the time duration * @return the converted duration in this unit, * or {@code Long.MIN_VALUE} if conversion would negatively overflow, * or {@code Long.MAX_VALUE} if it would positively overflow. * @throws NullPointerException if {@code duration} is null */ public long convert(Duration duration) { long s = convert(duration.getSeconds(), SECONDS); if (s == Long.MIN_VALUE) return s; long n = convert(duration.getNano(), NANOSECONDS); assert n >= 0 && n < 1_000_000_000; return (s + n < s) ? Long.MAX_VALUE : s + n; } /** * Converts the given time duration in this unit to a Duration. * * @param duration the time duration * @return the time duration represented as a Duration * @throws ArithmeticException if the duration cannot be represented * as a Duration due to numeric overflow */ public Duration toDuration(long duration) { return Duration.of(duration, toChronoUnit()); }