It is computed from what the contract is actually streaming — the live reward rate against the live total staked — not from a target, a projection or an emissions schedule. It moves when fees move and when the amount staked moves.
It is shown as APR, not APY, deliberately: rewards are paid in a different token than the one you stake, so there is nothing to auto-compound and quoting a compounded figure would overstate it.
Rewards are not minted continuously out of nowhere. Someone deposits a batch of reward tokens along with a window to spread them over, and the contract pays that batch out per second across the window. That date is when the batch deposited so far runs out — it is not an expiry, and nothing about it ends staking.
Topping up before the date does not restart or forfeit anything: the unstreamed remainder is rolled into the new window. If the date passes without a top-up the rate simply goes to zero until one arrives — your stake and everything you have already earned are untouched and remain withdrawable.
An APR in percent needs the value of one YRIS in reward-token terms. YRIS now trades on Pons, but not against this pool's reward token and not on a venue this page reads — so a percentage here would still rest on a price we fetched from somewhere we do not verify. The raw figures above are read from the staking contract and are exactly what a percentage would be derived from once there is a quote we are willing to stand behind.