The subjunctive "were" may be past tense but does not refer to past time. It is used to refer to an unreal present ("If I were you, I would do something about it") or unreal future ("If I were able to be there tomorrow, I would be").
Tense and time should not be confused. Tense is an inflectional category. "I fly to Paris on Monday" is present tense but refers to future time. In "If I had the money, I would give it to you", "had" is past tense (and would traditionally have been called "past subjunctive") but refers to an unreal present.
So, what neither of the existing answers quite says, but Colin Fine makes clear in the comments, is that although "historically were is "past subjunctive" (which for every other verb in the language is identical with the simple past)", "this does not mean that it has any hint of past meaning at all: it is simply a name given to this form, which is used for counter-factuals".
So in the traditional term "past subjunctive", the term "past" refers to form, not time. But the same is true when we talk about the past tense generally, the present tense, etc.
In the traditional view, which he alludes to, the past indicative and past subjunctive are identical for every verb except "be" (which has the "was"/"were" distinction): in other words, the two moods show an extremely high level of syncretism or overlap.
However, in informal English, people often say "if I was" where "if I were" is formally prescribed. So one of the disadvantages of the traditional view is that every time you see "if I had", "if I said", "if I went", you actually have no idea whether it's a subjunctive or an indicative being used in place of a subjunctive. Hence it makes more sense, instead of seeing the past subjunctive as a complete mood, to see the special use of "were" as an isolated "irrealis" mood-form specific to the verb "to be". Once seen as an isolated form, the question of tense becomes even less relevant.
