[Remind-Fans] Creating events that repeat every N days for a couple days

[Tim Chase]

I have a couple house-keeping chores that take me several days to do
and are supposed to fall on an N-day schedule but I'm having trouble
defining such a repetition in remind.  Ideally I'd get a "be
prepared" reminder for a day or two ahead (like "+2"), too.

My attempt to create some sort of "magic()" function has been
unsuccessful:

  SET BackupStart date(2019, 2, 5)
  SET BackupDuration 3
  SET BackupPeriodicity 90
  SET WarningDays 2

  REM [magic(BackupStart, BackupPeriodicity, 0)] \
    ++[WarningDays] \
    THROUGH [magic(BackupStart, BackupPeriodicity, BackupDuration)] \
    MSG %"Test backup cycle%" %b%

Alternatively, I'm not sure if I need to use a SATISFY
(maybe in conjunction with a FROM/SCANFROM).

I think the range of dates to consider/scan would be

  TrigDate - (1 + periodicity - duration)days

through

  TrigDate + (1 + periodicity - duration)days

checking if

  (TrigDate - StartDate)days mod periodicity

falls within the range of 0..(Duration - 1)

Anybody else here have tips on how to do something like this? 

Thanks!

-tim