I always thought that this expression in “Alice in Wonderland” was merely childish storybook stuff and nothing to do with anything learned. But recently I found on the Web that Lewis Carroll when writing “Alice in Wonderland” did not invent this expression, but took it from an old Latin grammar that used it as a rule to tell schoolboys learning Latin not to use “iam” for present events (for which instead use “nunc”) but only for past and future events.
In the well-known “Ave Caesar, adsum iam forte” = “Hail Caesar, by chance I am here already”, the tense seems to be present, but in the sentence, the speaker is already here, and so his arrival here was in the past.