Immutable way to cast timestamp TEXT to DATE? (for index)

2014-10-23T00:00:00

I'd like to be able to create an index on the date portion of this field (as a date), because I have lots of queries that are searching for particular dates or ranges.

I've tried various ways of getting to a date (::date, LEFT(x,10)::date, etc.), but all of them throw the error "functions in index expression must be marked IMMUTABLE."

Is there any way, hacky or otherwise, people know of to do this?  Thanks in advance.

Ken