Dynamic partial index

 Hello

 I'm interesting in a partial index for a rows that aren't older than 6 
mounts, something like the sentence below:
/    CREATE INDEX name_for_the_index ON table (the_column_of_type_date) 
        WHERE ( the_column_of_type_date > (current_date - interval '6 
month')::date );/

But this is not posible, I receive the next error:
    /ERROR:  functions in index predicate must be marked IMMUTABLE/

So, what is the best solution for my problem?

 Thank you,
       Gustavo