date question

Hi;
I've got a table in the way:

year | month | day | est7237 | est7238 | est7239 | est7250
------+-------+-----+---------+---------+---------+---------
2002 | 9 | 1 | | | |
2002 | 9 | 2 | | | |
2002 | 9 | 3 | | | |
...
This table is:
murciadb=# \d series_lluvia_completas
Table "series_lluvia_completas"
Column | Type | Modifiers
---------+---------+-----------
year | integer |
month | integer |
day | integer |
est7237 | real |
est7238 | real |
est7239 | real |
est7250 | real |

And I've got another related tables with a date field. I need to compare the
dates field in the other tables with "year", "month", "day" in this table.

Is there a way to create a table from this table, directly in Postgres in
which a date field is created based in the values of "year", "month", "day"
in this table?

Thanks and regards

Javier