A Straight Dope Classic from Cecil's Storehouse of Human Knowledge

The millennium approacheth. Will it start Jan. 1, 2000, or Jan. 1, 2001?

February 8, 1974

Dear Cecil:

Will the 21st century begin at twelve midnight of December 31, 1999, or 12:00:01 (midnight and one second) of January 1, 2000? Moreover, is midnight AM or PM? Does it belong to the day before or to the day to come? When is the first time that AM is used during the day? Or the last time that PM is used? Is the meridian referred to in AM and PM only at midnight, or is it at noon? Or both? Dave is really involved in this, so if you could hurry . . .

Cecil replies:

Tell Dave I said he ought to get himself a hobby.

The 21st century will start neither at midnight nor at 12:01 AM January 1, 2000. It will start on January 1, 2001. Our calendar starts with the year 1 (there is no zero); thus, the 1st century expired at the end of the year 100, and the 2nd century began at the start of 101. Similarly, the 21st century will begin at the start of 2001.

Strictly speaking, midnight belongs neither to the day before nor the day after, and it is not proper to designate midnight as either AM or PM. Midnight is the dividing line between days, in the same sense that the present is the dividing line between the future and the past. (Think about it.)

Midnight is defined in terms of noon--i.e., it's exactly 12 hours after noon. Noon, in turn (also called the meridian) is that instant at which the sun is at its highest point in the sky. (This, of course, depends on where you're standing. Because of time-zone standardization, when the clock strikes noon in most places the sun is usually a little ahead of or behind the meridian.)

The highest point in the sky is precisely that--a point; hence, it has no dimension, and the time that the sun is there has no duration. So "noon" is that unmeasurable instant during which the sun is neither ascending nor descending in the sky. (The ascension and descension are apparent, of course. You can never be too careful in this business.)

The first second of the conventional day comes after midnight-- 12:00:01, and is thus properly designated AM. Of course, if you like, you can divide time up into smaller units than seconds, in which case you could say, "1 nanosecond after 12, AM."  I hope you see the importance of the comma between "12" and "AM."

Part of the confusion you folks are having about time designations stems from our somewhat perverse system of using 12 numerals to count 24 hours. In 24-hour time systems, in which the last minute of the day begins at 2359, there is no real need for terms like noon, midnight, AM, or PM. Astronomers use such a system with noon as the beginning of the day, or 0000; in the more familiar military system, noon is 1200.

Which neatly illustrates (I hope) an important point about the way we designate time--namely, that it's pretty arbitrary. If we were to decide on December 31, 2000, at 1 nanosecond before midnight, that we'd prefer to call the coming century "the 22nd century" instead of "the 21st," we would change nothing but our own terminology. The calendar men might be upset, and the historians of millennia to come might be a little turned around, but the universe would hum on.

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