all the table values equal - Mailing list pgsql-sql

Hi list,
this is a Friday afternoon question... :)
I have a table like this:

id_user | value | datetime
1 | 1 | xxx
1 | 2 | xxx+1
1 | -1 | xxx+2
2 | -1 | xxx
2 | -1 | xxx+1
3 | 4 | xxx
3 | 10 | xxx+1

I'm trying to create a query that say me which id_user, inside a
interval of time, has all the values -1

The part if the interval it's not a problem, but I'm fighting with the 
aggregate...
Someone?

Thanks,
Michele