Finding the index of certain strings in a list

I'm actually using a dataset of much greater dimensions and length but I simplified my problem of string patterns in order to ask this question.

dates = {"2018-01-01 22:00:00", "2018-01-01 22:30:00", "2018-01-01 23:00:00", "2018-01-01 23:30:00", "2018-01-02 00:00:00", "2018-01-02 00:30:00", "2018-01-02 01:00:00", "2018-01-02 01:30:00", "2018-01-02 02:00:00", "2018-01-02 02:30:00", "2018-01-02 03:00:00", "2018-01-02 03:30:00", "2018-01-02 04:00:00", "2018-01-02 04:30:00", "2018-01-02 05:00:00", "2018-01-02 05:30:00", "2018-01-02 06:00:00", "2018-01-02 06:30:00", "2018-01-02 07:00:00", "2018-01-02 07:30:00", "2018-01-02 08:00:00", "2018-01-02 08:30:00", "2018-01-02 09:00:00", "2018-01-02 09:30:00", "2018-01-02 10:00:00", "2018-01-02 10:30:00", "2018-01-02 11:00:00", "2018-01-02 11:30:00", "2018-01-02 12:00:00", "2018-01-02 12:30:00", "2018-01-02 13:00:00", "2018-01-02 13:30:00", "2018-01-02 14:00:00", "2018-01-02 14:30:00", "2018-01-02 15:00:00", "2018-01-02 15:30:00", "2018-01-02 16:00:00", "2018-01-02 16:30:00", "2018-01-02 17:00:00", "2018-01-02 17:30:00", "2018-01-02 18:00:00", "2018-01-02 18:30:00", "2018-01-02 19:00:00", "2018-01-02 19:30:00", "2018-01-02 20:00:00", "2018-01-02 20:30:00", "2018-01-02 21:00:00", "2018-01-02 21:30:00", "2018-01-02 22:00:00", "2018-01-02 22:30:00", "2018-01-02 23:00:00", "2018-01-02 23:30:00", "2018-01-03 00:00:00", "2018-01-03 00:30:00", "2018-01-03 01:00:00", "2018-01-03 01:30:00"}

Assuming I have the above list of dates, I'm trying to find the index of the dates matching the below string pattern.

patt = ___ ~~ "23:30:00"

Both Positions nor Cases return empty with my pattern. StringCases does but the output is a bit unwieldily.

StringCases[dates, patt]

Which means I have to do something like below to get the indexes I need.

Select[   PositionIndex[    StringCases[ dates , patt ]    ],    Length@# < 2 &   ] // Values // Flatten(* Returns *){4, 52}

Does anyone know of a way to directly get the index of a string pattern with Position?