[Round Two O.C.T.O.B.E.R. XXIX] Modular

Modular arithmetic: also known as clock arithmetic.  The clock is actually what I will discuss today.

Most people, at least in the States, both write and speak on the twelve-hour clock (i.e. using AM/PM).  Because the most common users of the 24-hour clock in the States are in the military, the term “military time” is derived.  Most of the time, there is no ambiguity of the actual time using AM/PM, even if a qualifier such as “morning,” “afternoon,” “evening,” or “night” is not used.  The context of the sentence usually makes it clear, but not always!

I still often speak in the 12-hour clock, but occasionally will speak as if I were in the military.  But, my watch has been set on the 24-hour clock ever since sixth grade, and I have started writing exclusively in the 24-hour clock since early college.

In most other countries, clock times are written in the 24-hour convention, but people still vocalize times of greater than 12:00 hours as if they used the AM/PM convention.  For example, when I was in Israel, on some days when we had free reign over the area (e.g. Ben Yehuda Street), we were instructed to be back at “4.”  Of course, this meant the afternoon, but it didn’t confuse the Israelis either, despite all clocks saying “13:20” at that time.

My first experience seeing a 24-hour clock was when I visited the hospital when Papa was recovering from a heart attack.  The clock in his hospital unit read “17:30” when we walked in the room and I asked Dad about it.  Indeed, in certain settings, it makes sense to go with the least ambiguous clock.

Interestingly, the impetus for me for starting to write time in the 24-hour clock was when I played Legend of Zelda: Ocarina of Time.  The game’s time (not on the same clock as real life) is given as a 24-hour clock, as can be seen when you attack one of the “masky” rocks. (Link swings his sword at the rock.  Response: “BOING BOING! The current time is 18:06.”)

Granted, there were some evenings during that year when I would look at my watch, see the time (e.g. 19:22), and think that it was much later than I thought.  But then I remembered, “Oh, yeah.  Subtract 12.”

In either clock, there is the ambiguous time of midnight as to whether it corresponds to the previous day or the next day.  Therefore, many time-based objects avoid this ambiguity by either giving the time “11:59 PM” or “12:01 AM”.  In 24-hour time, that corresponds to “23:59” and “00:01”.  I’m not sure of when that would be particularly pernicious, but for example with deadlines, the deadline “midnight on October 31” leaves ambiguity as to whether that is past the limit of October 30 (i.e. 00:01 on October 31) or the limit of October 31 (i.e. 23:59 on October 31).

And one other reason I like the 24-hour clock: there is no ambiguity at noon, since I sometimes forget whether 12:00 PM corresponds to noon or midnight.  Right now, I do know that it means noon, but you never know when I could get confused for some reason.

I am not attempting to convert any 12-hour clock users to 24-hour clock users, however.


Today is the twenty-ninth day of O.C.T.O.B.E.R.  That makes four weeks and one day.


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