Can you save the Hardy reaction for a later turn, or does it apply automatically at 0 HP?

Question

Accepted Answer

It is optional: you choose to use it just before you would drop to 0 HP. You cannot use it while knocked out, so it cannot lift you from 0 HP on a later turn.

Arydia: The Paths We Dare Tread

Can you save the Hardy reaction for a later turn, or does it apply automatically at 0 HP?