Am I missing something, or does their solution not actually solve their specific example problem case at all?

Saving local time plus offset is _exactly_ the same information as saving GMT, isn't it?

You'd really need to save local datetime plus location... which they don't seem to actually be recommending?

Of course, for actually triggering an alarm or whatever, you'd still need to plot this on the actual timeline somehow, and deal with periodically checking to see if it should be changed because of a DST or timezone rule change or whatever.