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.