I am a choral singer who is slowly trying to learn German. I have a question about why the dative case is being used in some text from Johannes Brahms' Ein deutsches Requiem.
In particular, there is a movement with the following text :
Siehe, meine Tage sind einer Handbreit vor dir...
This comes from Psalm 39 and the phrase, as I understand it, would be roughly translated as:
Behold, my days are as a handbreadth before Thee...
I understand that this is a historical work (composed between 1865 and 1868), but I am confused why
eine Handbreit here is declined in the dative case (at least that's what I assume the
einer is marking). I understand that
vor in this situation takes the dative case, which is why
before Thee is translated as
vor dir, but why is
Handbreit also in dative? It seems like the nominative case would make more sense here, since we are using
sein to say
My days are...
At any rate, I assume I'm missing something, be it how
vor actually works, or this is some historical artifact.
 http://www.classical-music.com/article/brahmss-german-requiem-text (Full text, see movement 3 for the passage in question.)