The catch is that scrubs are expensive, especially when the pool holds petabytes. An admin who hit a power-supply problem or a sudden shutdown usually wants to scrub the data written around that event, not the entire pool. That is the goal here: teach ZFS something about wall-clock time.

Internally, ZFS only thinks in transaction group (TXG) numbers. Administrators, inconveniently, think in dates. A TXG is just a uint64_t that goes up with every transaction, and there has never been a clean way to map between the two.