Half-life decay vs time-dilation

If you have a substance that has a half-life of 1 day, for example, and send it off on a round-trip at 99.9% the speed-of-light for 1 day, when you get it back, from your frame-of-reference, 1 day has passed so only half of that substance should remain, but from the substance's frame-of-reference only about an hour has passed, so about 97% should remain. Which is right, if either?