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:

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    /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 );/

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But this is not posible, I receive the next error:

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   ERROR:  functions in index predicate must be marked IMMUTABLE

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So, what is the best solution for my problem?

Thank you,
      Gustavo