Date functions in WHERE clause? HELP

Question

The following two queries are identical except for how I supply the date values in the where clause, yet the first query using the a custom my_date function runs 30x slower than the one using the TO_DATE() function. Both return DATE types...any reason for the difference?
SELECT * from outcomes
WHERE start_time >=fn_my_date('START_MONTH')
AND start_time < fn_my_date('END_MONTH')+1
-- This runs 30x faster--
SELECT * from outcomes
WHERE start_time >=TO_DATE('08/01/2001','MM/DD/YYYY')
AND start_time < TO_DATE('08/31/2001','MM/DD/YYYY')+1
On the flip side, I've also experienced queries running slower using the TO_DATE() function vs the LAST_DAY(sysdate) for equivalenet dates.
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Answer 1

I haven't seen the message coming up when using LENGTH or SUBSTR, but every time I connect to a database or attempt to change my preferences (including the "NLS Parameters: Comp" preference), this appears.
You are attempting to set the preference you should be to switch this from ANSI to something else, but SQL Developer is ignoring the preference setting (which I think is a bug).
A way to set the NLS_COMP to something else is to use something like "alter session set nls_comp = BINARY;". Note that changing any preference after that appears to overwrite this and you need to do this for each new connection you start.