Hmm, not sure what you want here, but....
to define a field as a timestamp....
despatch_time datetime year to second
(or, perhaps a little simpler).
despatch_time datetime hour to second
....
When you select from the table and what to put criteria on it, you must use the datetime format specfied within the table.
First definition (datetime year to second)...
select ??? from ??? where despatch_time >= "2009-01-23 00:00:00"
Second definition (datetime hour to second)...
select ??? from ??? where despatch_time >= "16:45:00"
....
I *think* the default for datetimes is datetime year to fraction(4), but not 100% sure on that.