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