Get one data from each duplicate data row

I want to make select options to display the month taken from the created_at column of a table

// item_ins
+----+--------+---------------------+
| id |  item  |     created_at      |
+----+--------+---------------------+
|  1 | item 1 | 2017-12-11 10:37:52 |
|  2 | item 2 | 2017-12-11 10:38:17 |
|  3 | item 3 | 2018-01-12 01:28:43 |
|  4 | item 4 | 2018-01-12 01:30:14 |
|  5 | item 5 | 2018-02-12 01:30:05 |
|  6 | item 6 | 2018-02-12 01:30:42 |
+----+--------+---------------------+

in the table there are each two months december, january and february, and i want to get one from every same month, i try like this

$years = ItemIn::distinct('created_at')->pluck('created_at');

but I still get the same two months for each month because the timing is not the same, as below

and I'm still confused with how to get just a month because in the above way I get also years and days

[
    {
        "date": "2017-12-11 10:37:52.000000",
        "timezone_type": 3,
        "timezone": "UTC"
    },
    {
        "date": "2017-12-11 10:38:17.000000",
        "timezone_type": 3,
        "timezone": "UTC"
    },
    {
        "date": "2018-01-12 01:28:43.000000",
        "timezone_type": 3,
        "timezone": "UTC"
    },
    {
        "date": "2018-01-12 01:29:14.000000",
        "timezone_type": 3,
        "timezone": "UTC"
    },
    {
        "date": "2018-02-12 01:30:05.000000",
        "timezone_type": 3,
        "timezone": "UTC"
    },
    {
        "date": "2018-02-12 01:30:22.000000",
        "timezone_type": 3,
        "timezone": "UTC"
    },
]

via Chebli Mohamed