Hi, the concept of the length of generic periods puzzles me. The "normal" case p.length() = p.end() - p.begin() makes sense to me, but why is a period with last() == begin() treated as a special case, having length 0? Having this special case make the length concept behave strangely. For example, [2005-May-1/2005-May-1] has length 0, but [2005-May-1/2005-May-2] has length 2! Actually there is no period of length 1 at all. As I understand from the documentation, the philosophical reason behind this seems to be that the end points of periods are considered "points" and that a point and therefore a period that consists of a single point has zero extension. This sounds plausible if you think of the periods like intervals on the real number line. But our situation is different, I think, since we always deal with discrete quantities that have a finite length (or "duration"). I would interpret the period [2005-May-1/2005-May-1] as the time period that lasts during the whole day 2005-May-1, from 00:00 up to 23:59:59.999... Therefore, I would give it the length (duration) 1. [2005-May-1/2005-Apr-30] would accordingly be an empty period of length 0, and [2005-May-1/2005-Apr-29] would be an invalid period of length -1. The length of a date_period p would then be simply the number of days in p, multiplied with the unit of the duration_type. The template parameter duration_type can be used to defined different measurements of the length. In the standard case (duration of a day is 1) you measure the period length in days. But you could also measure it in seconds, by using a duration_type with unit() = 86400. What do you think about this? Sincerely, Friedrich