Before that observation (and importing that event into your application!), is it not semi-arbitrary, as far as the code is concerned? A fully defined point in the future (currently) corresponds 1:1 to a unix time, or a timestamp from this RFC, almost by definition.
If you can't calculate a delta from $now, or "are $X and $Y the same moment in time" , I don't think "timestamps" are the tool. Seems beyond the scope of RFCs like this, at least to me.
Without guaranteed internet access, how would you set an alarm for "this day next year"? Timezones changing already cause problems there, but those were least correct at one point - not a "quantum timestamp" of sorts.