Agreed this sounds like they are looking for Intl.DurationFormatter. Temporal.Duration.prototype.toLocaleString will end up using Intl.DurationFormatter though

Agreed this sounds like they are looking for Intl.DurationFormatter. Temporal.Duration.prototype.toLocaleString will end up using Intl.DurationFormatter though, so that will ultimately be what he wants (probably going to depend on that ICU4X implementation being complete though).