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When considering a set of days and writing about them in a 'diary mode', what preposition should I use in the following example:

  • He died at day 5 of the treatment.
  • He died on day 5 of the treatment.
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up vote 3 down vote accepted

"On" works better here. It implies that he died at some point on the 5th day of the treatment. I would use "at" to reference a very specific point in time (rather than a relatively longer period of time, such as a day), for example: "He died on the 5th day of the treatment at 3:02 PM".

See also: http://www.englishclub.com/grammar/prepositions-at-in-on-time.htm

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Thanks! I thought the same, but wasn't 100% secure :) – JS Mann May 16 '12 at 17:15
Perfect! You should delete "I think". +1 – user21032 May 16 '12 at 17:16

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