Re: [calsify] SKIP was Re: AD review of draft-ietf-calext-rscale-03

[Ken Murchison <[e-mail]>]

On 02/05/2015 11:13 AM, Barry Leiba wrote:
>>> I do NOT think that we need to accompany this with any list of
>>> calendars.  Whoever implements a particular calendar needs to know how
>>> it works in order to generate the right SKIP values anyway.  So why
>>> not just say that, and forget about having to include the SKIPs?
>> Because there should be an agreement between all implementation on what SKIP
>> default is applied to a given Calendar. Say, we agree here that SKIP should
>> default to BACKWARD for all calendars except HEBREW, for which it should
>> default to FORWARD. This information needs to be written somewhere and set
>> in stone, so that all implementation apply correct default when expanding
>> RRULEs.
> No, the point isn't that we should agree that SKIP defaults to
> BACKWARD or FORWARD or BANANA, except for anything.  We should agree
> that the way each calendar behaves in this regard is specific to each
> calendar, and implementors of a particular calendar need to understand
> how that calendar works and implement it right.
>
>> Otherwise an implementor without deep knowledge of a particular calendar
>> (its not required if he bases his implementation on ICU for example), can
>> apply a different default. I don't see how can we do without a registry for
>> variable defaults, if we choose this option.
> We do this sort of thing all the time, and it works.  Do we get bad
> implementations sometimes, because someone gets it wrong?  Sure.
> Those usually get fixed.  I think that situation is *far* better than
> having skip rules (perhaps wrong ones, created by the same buggy
> implementations) imbedded in the iCalendar entries.

FWIW, the ICU library, which I assume most implementations will use, 
seems to always skip forward to the next valid date if/when you try to 
set an invalid one, regardless of calendar.  So, If I try to set Feb 29 
in a non-leap year it gives me Mar 1.  If I try to set 30 Adar I in a 
non-leap year it gives me 1 Nisan. I could try some more examples using 
Chinese leap months and Ethiopic leap day.

AFAICT, it never skips backwards on its own, I have to do that one layer up.

-- 
Kenneth Murchison
Principal Systems Software Engineer
Carnegie Mellon University