A:What are you doing in the library? B:I'm studying for our science test. A:But it's not for another two weeks. 請問:最後一句的文法與用法,是否省略了什麼?

"But it's not for another two weeks."
≡ But it's not for another two weeks that this test will be held.
≡「但應該還有兩星期的時間」, 這測驗才舉行。
　　　　　　　　　　　　　　|←--省畧了--→|

這句省略了 " ... that this test will be held."
-- 因為是呼應前兩句的 conversation, ∴ 可以只寫出重要的字眼, 其他的當作 understood！

The verb seem is used in this pattern. If the infinitive is be with an adjective or noun as complement, to be may be omitted when the adjective is one that is used both predicatively, as in:-eg:-With introductory it;
It seems but ((the test is not (to be) held))------adjective clause;
for another two weeks------noun as complement;
According to your writing:-eg:-
But (the test is not to be held)----adj cl.
for another 2 weeks-----noun as complement;
=But it's not for another 2 weeks----n as complement
--(for the test to be held)---------this clause may be omitted; leave out.