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in 4.2.26. Date, it sez: "If the time value of V is NaN, then return the undefined IDL Date value." Does that mean "return undefined"? or "return new Date(undefined)"? or something else? It's not really clear to me how the following is represented: "An additional value that this type can represent is one that indicates an indeterminate or undefined time, which we write as undefined."
This is the algorithm from converting from a JS value to an IDL value of type Date. It doesn't mean "return undefined" or "return new Date(undefined)" -- it doesn't really make sense to think about returning JS values here. The IDL Date type has values that correspond to all of the possibly time values that JS Date objects can represent, including the time value that does not represent a specific point in time (which in a JS Date object is one that could be created by for example evaluating `new Date("")`). In space of IDL Date type values, I just call that value "undefined".