When does a poison interval of "one round" end?

Question

When does a poison saving throw interval of "one round" end?

Take this poison for example:

Hunting Spider Venom (poison) Saving Throw Fortitude DC 16; Maximum Duration 6 rounds; Stage 1 1d10 poison and flat-footed (1 round); Stage 2 1d12 poison, clumsy 1, and flat-footed (1 round); Stage 3 2d6 poison, clumsy 2, and flat-footed (1 round).

Let's say a spider hits me with an attack that delivers this poison. And let's say I fail my save.

If you fail the initial saving throw, after the affliction’s onset period elapses (if applicable), you advance to stage 1 of the affliction and are subjected to the listed effect.

Given that I wasn't previously poisoned, and given that my save wasn't a critical failure, I advance to stage 1, I take 1d10 poison damage, and I become flat-footed.

At the end of a stage’s listed interval, you must attempt a new saving throw.

When is this new saving throw performed?

• At the start of the attacking spider's next turn?
• At the end of the attacking spider's next turn?
• At the start of my next turn?
• At the end of my next turn?

I believe the third option (at the start of my next turn) is the correct answer, but I didn't find the rule that makes it so.

Answer 1

The poison interval occurs at the start of the attacking spider's next turn.

This comes from the Duration rules, specifically:

For an effect that lasts a number of rounds, the remaining duration decreases by 1 at the start of each turn of the creature that created the effect.

So at the start of the spider's turn, the flat-footed condition you were inflicted with ends and you make a new saving throw to determine if you advance to the affliction's next stage or if you manage to cure yourself.