How Can I get versions that they haven't released in the same month?

How Can I get versions that they haven't released in the same month?

I'm using this command to get the time that the versions of a project has
been released.
git log --tags --simplify-by-decoration --pretty="format:%ci %d"
And the output is something like this:
2013-06-08 09:11:40 +0200 (2.1.4)
2013-05-26 17:31:17 +0200 (2.1.3)
2013-05-26 10:21:49 +0200 (2.1.2)
2013-05-25 10:07:49 +0200 (2.1.1)
2013-05-23 22:47:11 +0200 (2.1.0)
2013-04-30 18:20:53 +0200 (2.0.11, origin/2.0_bugfix)
2013-04-30 10:47:51 +0200 (2.0.10)
2013-04-13 14:04:35 +0200 (2.0.9)
2013-02-17 20:15:22 +0100 (2.0.8)
So what should I do in order to just get the versions that they haven't
released in the same month? I would like to achieve this output.
2013-06-08 09:11:40 +0200 (2.1.4)
2013-05-23 22:47:11 +0200 (2.1.0)
2013-04-13 14:04:35 +0200 (2.0.9)
2013-02-17 20:15:22 +0100 (2.0.8)