The first line of the poem has nobody in it. "Causing papers to be entered and sorted" is a dangling phrase with no subject, and its passive "to be entered" makes the work sound as though it happens by itself. Only in the second line does an "I" appear, and it arrives to lose something: "I lose the afternoon." The order matters. Before the speaker can say who he is, he has already been absorbed into a process, and what he gets back is a loss.
That verb is well chosen. We lose keys, wallets and library books, not afternoons, so "lose" quietly turns time into an object that can be misplaced. The irony is that sorting exists to stop things going missing, yet the sorting is exactly what makes the afternoon vanish. "Causing" adds a small note of complicity. The speaker is not simply a victim of the paperwork, because he is the one making it happen. The first line's vocabulary, "entered" and "sorted", is the language of offices and files. Once the afternoon has been "lost", it sounds like one more item that has been filed away.
The hinge of the poem is the word of its title. "The sun is now over" can mean the sun's day is finished, and it can also mean the sun has passed across the sky. Because the poem uses no punctuation at all, the line break lets "over" lean forward as well: "over / the under day" reads as the sun hanging above something. The title makes the same offer. "Over" is a spatial word and a word for endings, and the poem needs both. The speaker looks up from his desk and finds the afternoon finished. Judging by the text, he never saw it go.
Then comes the strangest phrase, "the under day". It is not an ordinary expression, and its meaning is open. It could be the day lived under pressure, under a heap of paper. It could be the shadowed underside of the day, the part the sun does not reach. It could be a second, interior day running beneath the bright one outside the window. In each reading the sun is "over" it, either finished with it or shining above it without touching it. "Under" also answers "over" so exactly that the last two lines work as a pair, like a seesaw. Both words are two-syllable trochees (OH-ver, UN-der), so the rhythm mirrors the idea.
The shape of the poem backs this up. By my count the lines run to eleven, six, six and four syllables, so the poem shrinks as it goes, and the afternoon shrinks with it. The first line, long and slightly clumsy, with its stacked "-ed" endings, sounds like administrative prose. The next two lines are plain and mostly monosyllabic, almost conversational: "I lose the afternoon." The last line is the shortest, and it ends on "day", the biggest word in the poem, which has been cut down by the modifier "under". There is no rhyme and no capital letter or full stop, so the poem has no formal sense of an ending. What it has instead is a soft, recurring "-er" sound: papers, entered, sorted, afternoon, over, under. That sound sits in the ear like a low office hum and ties the bureaucratic first line to the closing pair.
The tone is flat, and the flatness is what makes the poem work. The speaker does not complain, exclaim or explain. He reports that time has gone, in the same even voice in which the papers get sorted, and the reader supplies the sting. That is a small trick, but it is a real one. A school student who has lost an evening to a screen or a pile of homework will recognize the feeling, and the poem lets us have it in just twenty-seven syllables.
Its significance lies in this compression. In four unpunctuated lines it shows how routine can pass for a whole stretch of life, and how a person can come out the other end of a task and discover that the sun has already moved on. The poem is hard to treat as a complaint, because it simply puts the loss in front of us, in a single double-edged word.