There are definitely plenty of other JS engines, but they aren't always up to date on newer JS features. I'm pretty sure this is the 3rd JS engine to fully support the Temporal API (even JSC hasn't shipped it yet).

More like 8th. These pass nearly all Temporal tests as well: v8, spidermonkey, libjs, boa, escargot, kiesel, jint. Almost there: graaljs, yavashark.