datetime arithmatic

Folks,

    I am new to postgresql 7.1 and like what I see so far.  How is datetime
arithmatic performed in postgres.  I have a table with start date/time
and end date/time(datetime data type) fields and want to set the default
end time to 1 hour later than the start time.  Is there a way to do
this.  My attempts to set the default value for the field using:

starttime datetime default now(),
endtime datetime default now() + xxx.xxx,

has yielded only either a datetime with time set to 00:00 or  only a
date!

Thanks for the help.

Stuart