"Day" from 8am to 3am

Hi folks,

I have a special application that considers a playday to be from 8am to
3pm the next morning.  The system then does maintenance from 3am till 8am
(if necessary).

I'd like to ease application development by creating a new kind of
"playDay" to query from.  For example:

SELECT * FROM table WHERE timestamp IN date_trunc('playDay', timestamp);

Any ideas?
Where should I begin?  Or is this all just wrongheaded and should I put it
on the application level?

-judd