Geoff's post pointed my attention to a different solution for my
problem.
Again, my issue was editing a person's age, which is stored as days
in the db, in a grid with 3 columns for years, months and days.
An alternative to calculating those 3 fields from the db field on the
client using IBO's calculated fields is to define a trigger which
returns those 3 fields instead.
To update the table, I can either use stored procedures and use them
in IBO's query, or (preferrable in my opinion) define triggers in
interbase to do the update, so the view looks like a table to IBO
(any anyone else - this would be the advantage).

Thanks for the (indirect) hint.

Oh, and Helen: I usually very much appreciate your help and opinions,
but dislike those replies where the bottom line is: you don't want to
do it the way you described. I would perhaps agree on this, but then
there is the end user...

Mirco