On Thu, 2005-07-21 at 21:01 -0700, Jeff Garland wrote: <skip>
For example, [2005-May-1/2005-May-1] has length 0,
Well, that seems incorrect [2005-May-1/2005-May-1) has length zero (Note the non-inclusive end). But, then again, what is the duration of [1,1] -- zero I believe.
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").
Actually, I don't think it is different from an 'integer numberline'.
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.
Yes, that seems correct except that a date-period cannot represent anything less than a single day. Just as integers in a range [1,1] cannot represent 1.99999.
Jeff
I'm somewhat confused and I fear we don't quite understand each other. Let me try to clarify. If you interpret the periods as subsets of the integer number line Z, so that date_periods are sets of days, you get into troubles, for it is unclear what the "openness" of the intervals should mean. For example, [1, 2) = [1, 1] = {1}, [1, 3) = [1, 2] = {1, 2}, and so on. Z with the usual distance metric carries the discrete topology, i.e., *every* subset is open and closed. You would get a consistent system if you define the length of [a, b + 1) = [a, b] as b -a (i.e., last() - begin()). Then, a single day period [a, a + 1) would indeed have length 0, and [1, 3) would have length 1 (not 2). What I had in mind is a different interpretation, and I believe it better captures what we have in mind when we use the periods. I consider periods as intervals on the real number line R. Now, a day is not a "point" on R, it is an interval itself. I would (as I said above) treat it as the right-open interval lasting from 00:00 to 23:59:59.999.... Similar, a second like 2005-May-1 10:13:30 is a right-open interval, lasting from 10:13:30.0 until 10:13:30.9999... The points of R are time instances with zero duration, whereas a second has a duration of, well, a second. What *is* a point on R is the moment at which a day starts. We have a little confusion here since in notations like [2005-May-1, 2005-May-2), "2005-May-1" does not refer to the whole day, but to its start, so it should be considered as a shorthand for 2005-May-1 00:00:00.0; likewise for 2005-May-2. In this interpretation, a date_period is is not a set of days, but a union of days. It lasts from the begin of its starting day to the end of its last day. It is a truly half-open interval, so [2005-May-1, 2005-May-2) is different from the closed interval [2005-May-1, 2005-May-2] (though they have the same length); it is also different from the closed interval [2005-May-1, 2005-May-1] which is a single point of length 0. The interpretation as half-open intervals is built into the semantics of date_period, so closed intervals cannot be expressed at all. [2005-May-1, 2005-May-1) can be expressed and is the empty set (of length 0). begin() returns the first day (not its starting point) contained in the date_period, and last() returns its last day (again, not its starting point). The length of a period can generally be defined as end() - begin() (not as last() - begin() as in the Z-interpretation above). It turns out that [2005-May-1, 2005-May-2) just describes the whole day 2001-May-1 and has length 1. [2005-May-1, 2005-May-1) is the empty set and has length 0. In this interpretation, time_periods (based on ptime) are likewise subsets of R. The difference is only that time_periods allow a much finer resolution. Every date_period could be expressed as a time_period. For time periods, we can ask if a certain microsecond (again, this is not a point, but a half-open interval of finite length) is contained in it; for date_periods, days are the smallest objects we consider. Sorry, this email became somewhat lengthy, but I do not know how to express what I mean with fewer words. Sincerely, Friedrich