Join three fields into one on same table

Hi, i need help for a query. I have three fields, year, month and day
into table and need join and update another field named date on same
table.

My problem is not that make update query.

With this query have the value of the update:
> select year || '-' || month || '-' || day || ' 01:00:00' as newdate from table

but this another query not have correct syntax...
> update table set date=(select year || '-' || month || '-' || day || ' 01:00:00' as newdate from table)

Regards,
Jordi