Why do we say "It's time we ate" and not "It's time we eat"?

Solution 1:

It's an example of the past subjunctive:

Past subjunctive

Like the term present subjunctive, past subjunctive can be misunderstood, as it describes a form rather than a meaning. The past subjunctive is so named because it resembles the past indicative in form, but the difference between them is a difference in modality, not in temporality. For example, in If that were true, I would know it, the word were (a past subjunctive) has no past-tense sense and instead describes a counterfactual condition in the present.

In addition to appearing in counterfactual if clauses (If I were there, he would know it), the past subjunctive form also appears in that clauses expressing a wish that is unlikely to be fulfilled. Usually the main-clause verb in this circumstance is wish, as in I wish that he were here now; but occasionally some other expression implying an unlikely wish is seen, as in It is high time (that) he bought a new car or I would rather that he did that. (The latter example can also be recast in the present subjunctive, expressing doubt but not as much doubt as the past subjunctive: I would rather that he do that).

Your examples are correct, and replacing them with simple present tense verbs wouldn't be standard usage.

To give the issue an extra spin, the article goes on to say that the present tense is also valid, with a slight twist in meaning:

In contrast, hoped-for things that may possibly occur take the indicative. In this circumstance, often the main clause verb is hope, as in I hope that he comes now. Likewise, in It is high time (that) he buys a new car, there is a real possibility that he will indeed do so.

FumbleFingers, in the comments below, questions the validity of the present tense construction. The outcome of our discussion (such as it was) was that it appears to be significantly less common (therefore either newfangled, dialectical, or both), and it is dubious whether most people would accept it. So my best advice is to take that paragraph with a grain of salt, and use the past tense if you want to use this type of construction.