On Sat, Apr 16, 2022, at 13:35, Peter J. Holzer wrote: > When adding a timedeltacal object to a datetime, the fields are added > from most to least significant: First a new date is computed by > advancing the number of months specified [TODO: Research how other > systems handle overflow (e.g. 2022-01-31 + 1 month: 2022-02-31 doesn't > exist)], then advance the number of days. Finally add the number of > seconds and microseconds, taking into accout daylight savings time > switches if the datetime is time zone aware. > > Subtracting a timedeltacal object from a datetime is the same, just in > the opposite direction. > > Note that t + d - d is in general not equal to t.
I'm not sure there's a guarantee that t + n day + m second may not be equal to t + m second + n day, either. This is possibly also something that we can look at what existing implementations do, though I'm concerned that the choice of "fold" rather than "isdst" may come back to bite us here [C actually uses a three-state isdst input for mktime which performs these operations] Also, what about hours? "12 calendar hours" may be 13 or 11 hours depending on if a DST transition is included in the given time. -- https://mail.python.org/mailman/listinfo/python-list
