The Ready action lets you react to a "perceivable circumstance." Trying to ready an action based on the initiative order is apparently possible, if you Ready based on a creature moving or attacking, rather than based on "their turn".
But what about real (in-world) seconds or milliseconds? Can a character say "I Ready the attack action against this hostage, preparing to bring my axe down on them in three seconds"? What if they count out loud and ready their action to trigger when they say the word "three"? What about five and nine-tenths seconds? What do any of these situations mean as far as turn order goes?