updating table where one date field is null

I'm passing some information to postges (from gambas basic), which gets the
data from date type

eg date1 as date
     date2 as date

(IN the postgres database the default on the date field is null)

my problem is that postgres baulks when I"ve not put a date into one of the
gambas date variables with the message:

"invalid syntax for type date '00:00:00'"

my sql is far more complex but reduced to the date part reads something like
this

insert into mytable (date1, date2) values

('01/01/2008','00:00:00')

I wondered if anyone could volunteer a solution

Regards

Richard