Postgresql partial index on date to query sub period - postgresql

Question

On some very big tables, one of the most common indexes are on date field (e.g. date of insert, or date of operation).
But when the table is very large, the index tend also to but large.
I thought I could create a partial index for each month on the table, so that the query on the 'current' month would be quicker.
ex.
CREATE TABLE test_index_partial
(
exint1 integer,
exint2 integer,
exdatetime1 timestamp without time zone
);
CREATE INDEX part_date_201503
ON test_index_partial USING btree
(exdatetime1)
WHERE exdatetime1 >= '20150301 000000' and exdatetime1 < '20150401 000000' ;
For my tests, I fed a table with the following instruction :
INSERT INTO test_index_partial SELECT sint1, sint2, th
from generate_series(0, 100, 1) sint1,
generate_series(0, 100, 1) sint2,
generate_series(date_trunc('hour', now()) - interval '90 day', now(), '1 day') th ;
When I perform a select with the exact condition, it uses the partial index. But after creating a 'all_dates' index, not partial, on the date field, it prefered the whole index.
What I wanted : if I select with a more narrow period (ex., a five day period), I want postgresql to get the tinyer index. In my example : I'd want him to choose the index of the month instead of the whole index on all the table.
But it seems Postgresql don't assume that the condition :
exdatetime1 >= '20150310 000000' and exdatetime1 <'20150315 000000'
is contained in the condition of the index
exdatetime1 >= '20150301 000000' and exdatetime1 <'20150401 000000'
I use postgresql 9.3, is it a limitation of partial index of this version ? or do I have other way to write it ?