Re: DTSTART for recurrence instances

Olivier Gutknecht wrote:

>
> On 3 déc. 03, at 21:55, Doug Royer wrote:
>
>>>
>>> There is also a note in 2445 4.8.4.4 about DATE value type that 
>>> actually tells something about DTSTART semantics in a RECURRENCE-ID 
>>> based component:
>>>
>>>  If the value of the "DTSTART" property is a DATE type value, then the
>>>    value MUST be the calendar date for the recurrence instance.
>>>
>>> The definition here is for the -recurrence instance- and is a MUST.
>>
>>
>> Yes and as the section and subject of that text is  'Recurrence ID', 
>> as in :
>>    If the value of the "DTSTART" property is a DATE type value, then the
>>     value [of RECURRENCE-ID] MUST be the calendar date for the 
>> recurrence instance
>
>
> Right, so we agree this is only about the 'value type' coherence 
> problem between (original) DTSTART & RECURRENCE-ID. 


Yes.

>
> On 4 déc. 03, at 22:10, Doug Royer wrote:
>
>> Craig Johnson wrote:
>>
>>> Olivier presented an excellent case showing that a RECCURRENCE-ID's
>>> primary role is that of an 'identifier'; and not a 'date/time' value 
>>> that can
>>> be relied on as the 'effective start' for a recurrence instance.  
>>> This is made
>>> even more clear in the following from 2445:
>>>  Section 4.8.4.4 Recurrence ID
>>> . . .
>>>    The date/time value (RECURRENCE-ID) is set to the time when the 
>>> original recurrence
>>>    instance would occur; meaning that if the intent is to change a
>>>    Friday meeting to Thursday, the date/time (RECURRENCE-ID) is 
>>> still set to the
>>>    original Friday meeting.
>>
>>
>> Meaning in the object being sent in the reschedule. In order to say X 
>> moves to Y,
>> you need to properties to hold X and Y. So the above paragraph means 
>> send a
>> new object with the new Y in RECURRENCE-ID and the old X in DTSTART.
>
> [..]
>
>>> With RECURRENCE-ID consigned to the role of an identifier
>>> it clearly falls to DTSTART to represent a recurrence instance's
>>> effective start time.
>>
>> 2445 is talking about a reschedule object not what 2446 calls the 
>> master object.
>
>
> I don't think so as the text only refer to recurrence instance 
> identification, and disagree with the above interpretation of having 
> the object sent to reschedule.


> If the RECURRENCE-ID value is not changed, "changes to the start/end 
> of the recurrence instance" are done, how can it be expressed if not 
> in DTSTART / DTEND values ? How can a rescheduled specific instance 
> could be expressed in a complete snapshot of the event state  if not 
> by having the recurrence-ID identifying the initial value for the 
> instance and dtstart giving the actual, rescheduled value ? 

As I posted the date math in a previous email (yesterday?)

>>> Doug wrote:
>>> > For a recurring instance the effective start of an instance
>>> > is defined in 2445 to be the RECURRENCE-ID.
>>>  That is not quite correct.  The effective start of an instance
>>> is its DTSTART.  RECURRENCE-ID gets DTSTART as its
>>> initial value:
>>>  4.8.4.4 Recurrence ID
>>> . . .                                               The property
>>>    value is the effective value of the "DTSTART" property of the
>>>    recurrence instance.
>>>  This statement makes it clear that a recurrence instance has a
>>> DTSTART property containing the "effective" value  ... and this is
>>> where RECURRENCE-ID gets its initial value.  Initially, DTSTART
>>> and RECURRENCE-ID have the same value.  If the instance is
>>> changed to a different time, DTSTART changes and
>>> RECURRENCE-ID does not.
>>
>>
>> Yet the topic of the section is RECURRENCE-ID and not DTSTART, so the
>> subject usage 'THE" in that sentence is RECURENCE-ID and not DTSTART.
>
>
> Immediatly below, this is precised to be "The date/time value is set 
> to the time when the original recurrence
>    instance would occur; meaning that if the intent is to change a
>    Friday meeting to Thursday, the date/time is still set to the
>    original Friday meeting."
>
> Why insisting on "the time when the *original* recurrence instance 
> would occur" if not to have a possible -distinct- dtstart ? the 'The' 
> in that case is the RECURRENCE-ID (topic of the section).


Again it is talking about a change object, not the original.

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