That's one reasonable interpretation, except that it causes problems where Jan 30 + 1m == Jan 31 + 1m and also Feb 28 - 1m != Jan 30 or 31.
Luckily most people can ignore leap seconds, just like pretty much every system time library. Because they don't happen on regular intervals it is impossible to code a fixed rule for dealing with leap seconds so very few things even attempt it.
Time is hard enough to deal with already and few human scale things care about 1 second differences that happen once every 3-5 years or so.
> what should Siri do if you tell it to "move today's 1 o'clock to the next month" ?
This is an interesting question, because there are at least two valid options. If it is the 31st of the month, then maybe you want it to happen on the first of next month. Something a human secretary might intuit. On the other hand, you might mean moving the appointment back a whole month, unless it's the 31st and then you mean to have it a day earlier on the next month...
Like I said, this kind of logic gets you in ambiguous edge case hell in a hurry. FWIW, I don't think Siri even attempts to deal with a request like that.
A better solution is probably to require the person to be a bit more explicit in their request "Siri, move my 1 o'clock to next Tuesday".
Besides, these kinds of interactions aren't impossible with epoch time, they just require more work. One can argue that it would be good to make this sort of thing a little tougher as it will encourage the programmer to think harder about what they are doing and reconsider if it is a good idea.
