The premise that sein is always followed by nominative is wrong. You don't even need a substantive, e.g. "Ich bin müde.".
You could say that sein is always accompanied by nominative; but seriously, there is (almost) always a nominative because the subject, which is obligatory, is in nominative. And in case someone raises objection to this statement: yes, there are some exceptions (e.g. "Mir ist kalt."), but this is irrelevant to your question.
So, sein is a verb that has quite a few definitions. This verb can even be used as copula, auxiliary and full verb.
One definition basically expresses an equality between two things. A sentence like "Ich bin Lehrer." means, kinda, 'I equal teacher'. In that case both parts have to be in the same case. Since I is in nominative (as it is the subject of the sentence), Lehrer is in nominative, too.
And this applies to your first example.
Another definition for sein is that something occurs at a certain place or at a certain time. Such a sentence doesn't have a second substantive whatsoever, so obviously there can't be another one in nominative. Your other example (i.e. "Es ist nächsten Sommer.") fits into this group.
The past form of that sentence (i.e. "Es ist letzten Sommer gewesen") is comprised of two sein. The former one is the auxiliary verb for the present perfect, the latter one is the past participle of sein, expressing yet again that something occurs at a certain time. There's no second substantive, hence there's not another nominative.
The full story about sein is sheer endless. But if I haven't missed anything, the only case with two nominatives is the equality that I talked about above. So, only when you can apply 'A equals B', the substantive following sein is in nominative.