I like to group this thus:
"It's time" followed by the past subjunctive indicates that the speaker thinks that the action should have already happened;
"It's about time" is more emphatic; and
"It's high time" is more emphatic again.
I think that "it's time and "it's about time" can also be followed by "to" and the infinitive, but "it's high time can't.
At the moment my Venn diagram doesn't contain preparatory phrases, In an effort to keep things simple, I've held off from putting them in before except in some of the links, but apart from the dummy "it", "here" or "there", they are essentially verbs followed by other verbs. Perhaps I should include them.
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