[Ronald Oussoren <ronaldoussoren@mac.com>]
IMHO “+ 1 days” and “+ 24 hours” are two different things. Date arithmetic is full of messy things like that.
But it's a fact that they _are_ the same in naive time, which Python's datetime single-timezone arithmetic implements: - A minute is exactly 60 seconds. - An hour is exactly 60 minutes. - A day is exactly 24 hours. - A week is exactly 7 days. No context is necessary: those are always true in naive time, and that lack of mess is "a feature" to those who accept it for what it is.
“+ 1 month” is another example of that (which the datetime module punts completely and can be a source of endless bikeshidding).
Note that the only units timedelta accepts have clear (utterly inarguable) meanings in naive time. That's intentional too. For example, "a month" and "a year" have no clear meanings (as durations) in naive time, so timedelta doesn't even pretend to support them. Despite all appearance to the contrary in this thread, naive time is bikeshed-free: it's easy for someone to know all there is to know about it by the time they're 12 ;-) datetime + timedelta(days=1) is equivalent to datetime + timedelta(hours=24) is equivalent to datetime + timedelta(minutes=60*24) is equivalent to datetime + timedelta(seconds=60*60*24) is equivalent to datetime + timedelta(microseconds=1000000*60*60*24) Naive time is easy to understand, reason about, and work with. When it comes to the real world, political adjustments to and within time zones can make the results dodgy, typically in the two DST-transition hours per year when most people living in a given time zone are sleeping. How much complexity do you want to endure in case they wake up? ;-) Guido's answer was "none in arithmetic - push all the complexity into conversions - then most uses can blissfully ignore the complexities". And note that because DST transitions "cancel out" over the span of a year, the benefits and the few dodgy cases don't really change regardless of whether you add one week or a hundred thousand weeks (although there's no way to predict what governments will decide the local clock "should say" a hundred thousand weeks from now - it's only predictable in naive time).