Wikipedia
In linguistics, T (always written as uppercase T plus uppercase EVL in subscript) is an abbreviation for the time of evaluation, a secondary temporal reference in establishing tense. It is used determining tense of generalizations and habitual truths. In these types of utterances, there is no specific verb occurrence to observe (and thus no assertion). Instead, the purpose of such utterances is to merely inform. In these utterances, a generalization or an habitual truth is attested as true (or questioned for trueness in interrogative forms). The earliest point at which these attestations can be evaluated as true or not serves as the secondary temporal reference for such constructions.
Grammatical tense represents the contrast between two measurements along the timeline of an utterance, with one of those measurements being the time of utterance T (the time at which the actual utterance is made). T is always the primary point of reference for tense. There are three references to which T can be contrasted: T — the time of assertion, T — the time of completion, and T — the time of evaluation; these are secondary references. The type used for the secondary reference is determined by aspect and type of utterance.