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I wrote the following article to Bubblews writing site (now gone) in 2014 …

The following article is based on a post I wrote to the Helium (writing site, now also gone) forum a couple of years ago. There had been discussion of exactly what Helium meant by midnight on a particular day, say the 14th of the month, as a deadline to request payment. Was it the start of the day or the end of the day? 

In lay terms, midnight is when the clock reads 12:00 am. It can be a tricky concept though. Midnight can be construed as the start of a day, or the end of a day, or both.  

International Standard ISO 8601 states: “As every day both starts and ends with midnight, the two notations 00:00 and 24:00 are available to distinguish the two midnights that can be associated with one date. (http://www.cl.cam.ac.uk/~mgk25/iso-time.html) 

Similarly at Wikipedia, “Midnight marks the beginning and ending of each day in civil time throughout the world. It is the dividing point between one day and another.” (http://en.wikipedia.org/wiki/Midnight) 

However, Greenwich 2000 Ltd states: “Every day starts precisely at midnight and A.M. starts immediately after that point in time e.g. 00:00:01 A.M.” (http://wwp.greenwichmeantime.com/info/noon.htm) 

US Government Printing Office Style Manual says: “12 a.m. (12 midnight)” (http://www.gpo.gov/fdsys/pkg/GPO-STYLEMANUAL-2008/html/GPO-STYLEMANUAL-2008-11.htm). In other words, they interpret midnight as being at the start of the day. They used to have midnight at the end of the day.  

See http://en.wikipedia.org/wiki/12-hour_clock#Confusion_at_noon_and_midnight (you have to copy and paste this one into the website bar) for various interpretations of midnight and the confusion it can cause. 

At Yahoo Answers, to the question “When is Tuesday midnight?”, the “best answer” says midnight is at the end of the day: “You are correct, midnight ends the day, Tuesday 1200 at night ends Tuesday, 1 minute later (or 1 second later, for that matter) is Wednesday. Your landlord’s note meant the period starting Tuesday Aug 1st 12:00:01, i.e. one second past Monday midnight, up to and including Tuesday Aug 8th 12:00:00, i.e. Tuesday midnight. Hope that helps.” (http://answers.yahoo.com/question/index?qid=20060806123904AAcCpih 

I’m not sure what happens in the intervening period of 1 minute or 1 second. Also, this “best answer” describes what is an eight day period (i.e. the start of Tuesday in one week to the end of Tuesday in the next) rather than a week, but the landlord’s note referred to a one week period. Nevertheless Asker chose this as the best answer out of 15, stating: “Good, concise, easy to understand answer. Most of the others added to the confusion.” The other 14 answers were a mixture: end of day, start of day, both, and for some it wasn’t clear. It was the same story on a few other forums I looked at. There is plenty of confusion.

I suppose it depends on whether you think of midnight as a moment in time (albeit an infinitely small one) or a concept that divides two days and therefore isn’t actually a time period or moment in time itself. If the latter, then midnight is at the start and the end of a day. If the former, then it could be either the first instant of a day or the last instant. 

Where time matters, it is probably best not to use the term midnight at all. Insurance contracts and transport timetables will usually use 11:59 pm (or 23:59) or 12:01 am (or 0:01) on a particular day rather than midnight to avoid confusion. A long time ago, I bowled in a few all night bowling marathons for some reason and the advertised check in time was usually given as 11.59 pm.  

I’m sure you can think of other examples of where midnight was used as part of a due date or a start time and you weren’t sure which day it meant. It’s best not to use the term in such situations.