1

I often use Google hit counts as a rough measure of how valid an expression is. In the case of "no later than" vs "not later than", Google finds 25 vs 12 million hits, indicating that both forms are probably ok. One secondary source I found claims that the first is colloquial, while the second is mostly used in documents of an official nature (this would explain why it is common, but not as much as the first form).

I would prefer a more authoritative reference for this claim, and promise to accept a valid answer not later than a week after it is posted.

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    What were the sources? We might be able to comment on their validity for a particular use. The Google Books NGram shows a less dramatic ratio than straight-up Ghits do, though writers clearly favored not later than for some time. – choster Jan 5 '16 at 20:48
  • Thanks for the Ngram link... (lost half hour of my life playing). I did not keep the source, but that is irrelevant, because it more or less quoted the write-up mentioned in the accepted answer. – yrodro Jan 7 '16 at 16:28
  • Hi, what does sth should be done "no later than November" mean? It means before 11.1 or 11.30? Thanks. – Bob Oct 3 '16 at 1:17
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Merriam-Webster published a write-up explaining the distinction, written by Jane Mairs, Director of English Language Learning Publishing.

No later than is used more often than not later than, and it is less formal. Not later than is used mostly in formal documents, such as rulebooks, government laws, and academic papers.

This supports exactly what you've said.

No later than examples:

  1. I’ll be back no later than 6 o’clock.
  2. We'll need to know your decision no later than next week.
  3. Cucumbers should be planted no later than August 31st.
  4. Republicans say they are going to vote no later than January 6th or 7th.

Not later than examples:

  1. All horses to be entered in the race must have a physical exam not later than 14 days before the race.
  2. In the legislature, a resolution with more than sixty co-sponsors asked the President to withdraw as soon as possible, but not later than October 1st, 2006.
  3. Not later than December 31, 2013, a health plan shall file a statement with the Secretary’s office, certifying that the data and information systems for such plan comply with all standards.
  4. The critical review shall be on a published scientific paper chosen from a list of papers announced by the Chairman of the Faculty of Biology not later than the second Friday of the winter term.
  • "All horses to be entered in the race must have a physical exam not later than 14 days before the race." (Example sentence is from Merriam-Webster as linked above; Kyle did not make it up.) Did the writer intend to say "...not earlier than 14 days before the race"? If not, what does the sentence mean? (Please forgive me if I'm just exceptionally dense today.) – Mark Hubbard Jan 5 '16 at 22:16
  • Good question. I disagree with your interpretation. I interpret the meaning as: The last day that horses can have a physical exam is 14 days before the race. – Kyle Jan 5 '16 at 23:55
  • Accepted as promised. This is what I found in a different page, but without attribution. I am happier to have a trustworthy reference for the explanation. – yrodro Jan 7 '16 at 16:34

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