I didn't plan on using a database at all whether for storing user data
or user/application settings. If the DOM becomes unwieldly, and SAX
can't marshal new XML documents then what should I use if I want to
stick with XML? I think I want to stick with XML because I thought it
was the modern technology to use for not only storing data but
configuration settings.

In the past I've used the Java Preferences API and it was great for
easily retrieving, manipulating data, and then storing changes but the
changes weren't portable nor editable without my custom GUI which
provided both edit and import/export features. The Preferences API is
just for settings and not for data so I figured I'd have to look to
something else this time.

The XML file I plan to use is going to store not only the properties of
a task (name, description, frequency, etc.) but also all the dates on
which the task will occur (based on frequency) which is what I would
consider the data in this situation. When the application starts up, and
just before the current month is displayed, I build a data structure
that contains all the dates for all the tasks on a month by month basis
(the biggest timespan displayed by my GUI so far) so every time a month
is displayed in the calendar all the tasks and all their occurrences are
displayed. I also plan to have a list-like view of a single task, all
its occurrences, and the status for which occurrences of that task have
been completed.

A user may modify the frequency of a task, complete a single or multiple
occurrences of a task, or create/delete a task and I was hoping an API
was available for me to easily handle data elements, that were sourced
from an XML file, to accomplish those user actions.

That's what I have in my head so far for designing this. Note that this
is just a for-fun project I thought I'd try for my own benefit so at
this point there is no need for it to be enterprise-ready (i.e.
multi-user, etc.).

At this point the calendar displays better than I imagined and I'm
trying to wrestle around how to compute, store and display multiple
occurrences of a single task, hence this thread.

If this was a professional project I would but I'm not worried about
that level of sophistication for something like this although your
example of a query probably isn't out of the question but it wouldn't
return any more than a handful of results for a user like myself or a
family member who might benefit from my program.