I am looking for an add-in or macro, or advice on how best to program
one, that
can do something like this:
(Assuming a series of past appointments it can locate and use) I enter
in an appointment for a certain day; it creates the appointment, checks
a certain number of past appointments, takes an average of the number
of days between them, and creates a new appointment that many days in
the future.

it would be nice if I could also then, shift the date of that future
appointment and have it repeat the process rather than having to make a
new corrected one and delete the old one.

I tried a recurring appointment set for a certain number of
days, and just alter each instance to reflect the correct date as it
occurs. But this means that the recurrence doesn't take those changes
into account. (For example, I set it to recur every 30 days, say, and
that puts an instance on the 6th. but it actually ended up taking
place on the 5th - even if i move it, the next one will be made 30 days
from the 6th, instead of where i want it, 30 days from the 5th.)

The important parts of this are the record of past items being accurate
and accessible, and being able to have accurate 'predictions' of the
next future event based on the average.

I'm currently doing the calculations in Excel, but I have to redo them
each time (when I add a new row it throws all the formulas off). Maybe
this would be better as something that runs in excel and then just
creates the appointment?

One more thing: however it's done, it needs to be in a format that can
sync to PocketPC 2003 once it gets into Outlook. (i.e. calendar and
tasks, but not journal.)

Any suggestions?