Where are we in "Ars Poetica"? Who knows? Suffice it to say we're in a metaphysical world of poetry, art, and the quintessential essence of what it means to "be" rather than mean. But don't get too confused that we can't really pin MacLeish's poem down to a specific time or place, and that's kind of the point.
If good poetry is timeless, then of course our speaker wouldn't look to give us any specifics. We've got a moon, a globed fruit, some birds, and a few lights above the sea, so really we could be anywhere.
Hey, so can poetry. Since the moon is visible everywhere on earth and it usually looks all groovy and otherworldly floating in the sky, we kind of feel like all of these images reflect the mystery of poetry and life. If anything, we're in the sort of poetic atmosphere that hovers over all of us and in effect unifies all of mankind, no matter a person's time or place. And good poetry should seek to do the same, according to our speaker.
So if the setting sounds as if it's all over the place, it's for a reason. And that reason is to help support the speaker's idea of poetry being motionless and timeless because it touches a part of us that can never be affected by time or space—hence the metaphysical vibe we get in "Ars Poetica."