CREATE TYPE

Can someone point me at some detailed instructions for creating new 
datatypes..

I've found quite a few web pages that mention it (in passing) and give brief 
examples - but nothing much I can actually work with for my purposes..

Ideally I'd like to use C as the language and the datatype will need an 
'extent' (like 'char' can be char(10) - although its nothing like a char 
field...)

I'm targetting this at the new 7.4 - so I think i need to use 'version 1' 
method (using Datum etc ?)

Also - the datatype itself is a comlex type which stores half a dozen 
different integers (Its a modified datetime - storing the year, month, day, 
hour, minute, second, but the extent gives it the ability to do YEAR TO DAY, 
HOUR TO SECOND etc, so you only get/set the relevant sections).
When 'selected' it would return a variable length string containing the 
relevant data, and would be set by passing in a string (some of the data of 
which may well be ignored if its outside the extent of the column etc)

Hope thats enough - all pointers greatfully received..
(and free'd when required :)