>Most of the time I'm using FDoY() to create an initial value for a date box, true, but sometimes i need it (and NDoY() more often) in a completely non-visual calculation routine.

I wouldn't say that whether they're used in a visual or nonvisual context matters when it comes to deciding whether a group of related functions belong to a single class.

>Keeping it as UDF saves me the overhead of CreateObject()ing a DateBox, just to call its methods as UDFs.

"Premature optimization is the root of all evil." :) And I wasn't suggesting a (visual) DateBox class, but a nonvisual Date class.