A year is always 365 or 366 days.
OK, some years have a leap second, but that is determined by a formula.
OK, but the Earth Rotation Service publish the information about leap seconds way more than a few months in advance.
All countries that switched to Gregorian calender did that at the same time.
Ok, all countries that existed at the time did it at the same time.
But they at least did it by scrapping the same days in a year.
No country will ever have DST and non-DST simultaneously
Well, at least not in a single timezone. (Lebanon).
The number of seconds in a duration of a fixed length between two timestamps are always the same, independent on when the duration is measured.
