Re: [calsify] iCalendar recurrence rule issue: Cannot describe offset relative to base rule

[Ken Murchison <[e-mail]>]

Hi Hadrien,

What you want to describe CAN be done with current RRULE syntax, just 
not as elegantly as what you propose.
You will find similar syntax in several VTIMEZONEs.

FREQ=MONTHLY;BYDAY=SA;BYMONTHDAY=13,14,15,16,17,18,19

FREQ=MONTHLY;BYDAY=TH;BYMONTHDAY=18,19,20,21,22,23,24


On 2/19/24 11:43 PM, Hadrien Grasland wrote:
> Hi,
>
> I am reporting this by e-mail because the "issue tracker" link in
> https://datatracker.ietf.org/wg/calext/about/ is dead. You may want to
> fix it.
>
> As a user of iCalendar-based applications, one shortcoming of recurrence
> rules which regularly bites me is that I may have a recuring event which
> is well described by the existing recurrence rules (e.g. every 3rd
> monday of the month => FREQ=MONTHLY;INTERVAL=1;BYDAY=3MO), but then I
> need to schedule a preparatory/cleanup step some time before/after this
> main event (e.g. the Saturday before, the Thursday after).
>
> The existing recurrence rule vocabulary cannot describe the recurrence
> rule associated with these preparatory and cleanup steps well. It may
> seem like in the above example, BYDAY=2SA would fit the "Saturday
> before" use case well, but it actually doesn't work because on some
> months, like the upcoming March 2024, the first Saturday of the month
> does not fall on the same week as the first monday of the month.
> Symmetrically, a "day X of week N+1" approximation for a positive offset
> rule would not work when the base rule is closer than a week to the last
> day of the target month on some months (usually February).
>
> This sort of relative recurrence rule would be better adressed by a
> supplementary recurrence rule field which enables one to offset the
> result of the recurrence rule computation by a certain number of days.
> In the above example, an extra OFFSET field like
> FREQ=MONTHLY;INTERVAL=1;BYDAY=3MO;OFFSET=-2D could handle the "Saturday
> before" use case, and an OFFSET=+3D variation could handle the "Thursday
> after" use case.
>
> Although the above strawman syntax leaves room for specifying offsets in
> units other than days, I'm not sure if all units would make sense. Weeks
> are fine (just multiply by 7), but months would be a harder to spec out
> (if I have scheduled something a month before the 30th of March, should
> the occurence be dropped or shifted?). Speaking for myself, a day-only
> offset field would handle every use case I've had so far, so such
> expressivity may not be necessary.
>
> Cheers,
> Hadrien
>
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-- 
Kenneth Murchison
Senior Software Developer
Fastmail US LLC