mandative - "It's time something was / were done about the problem."

It's time something was done about the problem.

It is a sentence from Murphy's "Grammar". Using was confused me, for me it should be were. It looks like this clause is in subjunctive mood—something that is unreal / imaginary, and in that case you always use were for any object.

Unless, of course, it is not the subjunctive mood, what would knock me down.

Answer

As @tchrist's Google Ngram shows, it is established idiom, beyond the reach of mere grammatical logic.

And anyway, it's mandative; if it were subjunctive, it should be present subjunctive, It's time something be done.

ADD:
Jespersen, A Modern English Grammar on Historical Principles, Vol 4 (I omit notes indexing citations.):

9.6. After it is (high) time it is usual to use the preterit. Originally this was in the subjunctive because it was looked upon as hypothetical, but as in the majority of instances there was no formal difference between the subjunctive and the indicative, and as the hypothetical element was not so clearly before the mind as in conditional sentences, the indicative came to be used.