[Tim, to Guido]
Do you really intend that we use all three: "classic arithmetic", "human arithmetic", and "strict arithmetic"?. If so, I don't grasp the intended distinction between "classic" and "human".(both seem to be the same as what I've been calling "naive arithmetic": the arithmetic Python currently implements for binary arithmetic operators involving at least one datetime object).
I _think_ I've divined the intent now: - "classic arithmetic": what Python datetime arithmetic currently does - "strict arithmetic": aka timeline arithmetic, what Lennart wants - "human arithmetic" aka "calendar operations" - including at least relative deltas involving units (like months and years) which have no fixed meaning in naive time This intersects with "classic arithmetic" (current "datetime +- timedelta"). but unlike those cases of classic arithmetic is intended to cover both the "naive" and "UTC timeline" models of time. For example, there "should be" a way to spell "an hour from now" that follows "strict arithmetic" rules when the datetime is aware. Or not ;-)