We discussed this a bit earlier, e.g., in #289 (comment) , but I wanted to open a separate issue to come to a conclusion on this one detail.

@sffc proposed using symbols stored as static properties, following the protocol proposal. I'd suggest using simple strings, as the protocols proposal has not been adopted, and strings are used for methods like this that already exist. I don't think we have to worry about namespace collisions here, which motivate some symbol use, and I think methods like Temporal.Calendar.prototype.plus are meaningful.