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

[Hadrien Grasland <[e-mail]>]

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