I never believe google translate.
Za literally means behind like on this picture http://www.k5learning.com/sites/all/files/prepositions.jpg
It can mean many other things.
Then I think "past" just about suffices as a translation. But "past" in a location sense (as opposed to a time sense) mainly means BEYOND not behind.
An example of how "past" and "behind" can have 2 different location meanings in English. I am standing on a street outside house #1. I ask you "what is PAST house #33?" the correct answer is "house #35, and the rest of the houses up to the end of the street". I ask you: "what is BEHIND house #33". The correct answer is "the back garden of house #33".
Here past and behind have two different meanings. Past will only mean "behind" in a subset of behind's meanings.
In this meaning za can be past/beyond. I did not think about it because in my mind it is "behind" from a speakers perspective. http://dictionary.cambridge.org/dictionary/english-polish/past_3
also learning za=past may not be good, as I think past may often used as "after" = po, and za with time means opposite ( to, in)
Is "biurkiem" taking the Instrumental Case because of "jest"? So "za" does not affect the object in this sentence?
preposition always dictates the case. za can be followed by accusative or instrumental. If it's direction- it's accusative, if it's location it's instrumental, if it's not literal you need to learn phrases with preposition and case.