Add period and interval concepts

[manuel-woelker]

This is a proposal to add period and interval concepts like those used in Joda Time
cf.
http://www.joda.org/joda-time/key_period.html
http://www.joda.org/joda-time/key_interval.html

Those are distinct from but interoperable with the existing duration type, and make it possible to describe operations like "add 1 year to this date" (this might be a different number of seconds depending on the date)

Is this something that you would consider for this library?

[lifthrasiir]

Not an explicit non-goal for Chrono but I wouldn't pursue that neither.

One big reason is that while Chrono closely follows ISO 8601 and other existing practices whenever applicable, there is no particular concensus about the non-regular duration arithmetics (ISO 8601 is a representation standard and does not define, say, the result of 2015-08-24 plus 1 month---it only standardizes that "an interval from 2015-08-24 to 2015-08-24 plus 1 month" can be alternatively written as 2015-08-24/P1M).

Implementing that makes Chrono highly opinionated and I don't think that really justifiable. Such arithmetics should go into a separate library depending on Chrono; I wouldn't object but I don't plan to write one.

[pickfire]

Isn't this in https://en.wikipedia.org/wiki/ISO_8601#Time_intervals?

[quodlibetor]

Yes it is. I think Intervals will probably be part of the larger iterator story in the upcoming refactor. This is also related to #245