On 2015-07-27 15:46, Lennart Regebro wrote: > On Mon, Jul 27, 2015 at 4:13 PM, Steven D'Aprano <steve at pearwood.info> wrote: >> To me, Paul's example is a datetime operation: you start with a datetime >> (7am today), perform arithmetic on it by adding a period of time (one >> day), and get a datetime as the result (7am tomorrow). > > Well, OK, let's propose these wordings: It looks like a date > operation, ie, add one to the date, but in reality it's a time > operation, ie add 86400 seconds to the time. These things sound > similar but are very different. > > I called it a "calendar" operation, because these operation include > such things as "add one year", where you expect to get the 27th of > July 2016, but you will get the 26th if you use a timedelta, because > 2016 is a leap year. So we need to separate date (or calendar) > operations from time operations. The same thing goes with months, add > 30 days, and you'll sometimes get the same result as if you add one > month and sometimes not. > Also, if you "add one year" to 29 February 2016, what date do you get? > timedelta adds time, not days, month or years. Except when you cross a > DST border where it suddenly, surprisingly and intentionally may add > more or less time than you told it to. >
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