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

Hi. I've got a text field in a table that holds this style of timestamp:

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

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