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2.7 — The Calendar and the Clock Themselves
The calendar is the strangest object in daily use. It has months named after the wrong numbers, a leap rule with three clauses, a week that matches nothing in nature, and eleven days that never happened.
Why the months are the shape they are
Why is September the ninth month when its name means seventh?
Because it was the seventh, and the year used to start in March.
The early Roman calendar had ten months, beginning with Martius (March, after Mars) and running to December. The names of the last six are simply numbers: septem, octo, novem, decem — seven, eight, nine, ten. September, October, November and December are still called by their old positions, two months out of step with where they now sit.
The remaining sixty-odd days of winter were not in the calendar at all. There was no month for them; the year simply stopped and restarted when farming did. Numa Pompilius, traditionally the second king of Rome, added January and February to fill the gap. When the start of the official year later moved to January — for administrative reasons, since consuls took office then — the four numbered months slid out of alignment and were never renamed.
The other names: January from Janus, the two-faced god of doorways who looks both ways; February from februa, a purification ritual held that month; April probably from aperire, to open, as buds do; May from the goddess Maia; June from Juno. July and August are the two that were renamed, for Julius Caesar and for Augustus.
Why does February have 28 days?
Because it is the month that got shortened, and it was shortened because it was last.
When January and February were added to the end of the old ten-month year, the calendar had to be forced to fit a lunar-ish scheme with an odd number of days in most months — Romans considered even numbers unlucky. Something had to absorb the remainder, and February, being the final month of the year at the time and associated with purification and the dead, absorbed it.
The story that Augustus stole a day from February to make August as long as July is a fabrication, first appearing in the thirteenth century. February already had 28 days under Julius Caesar's reform, and August already had 31. The lengths of the months come from Caesar's redistribution in 46 BCE, not from imperial vanity.
Why is there a leap year, and why the complicated rule?
Because the Earth does not take a whole number of days to go round the sun. It takes about 365.2422 days — the tropical year, measured from one spring equinox to the next.
Ignore the fraction and the calendar drifts by about a day every four years, so within a few centuries the spring festivals are in winter. Julius Caesar's fix in 46 BCE was to add one day every four years, treating the year as 365.25 days long.
That overcorrects by about 11 minutes a year, because 365.25 is slightly more than 365.2422. Eleven minutes is nothing in a lifetime and about three days every four hundred years — and by the 1500s the accumulated error was ten days, which the Church noticed because Easter is calculated from the spring equinox and the equinox was no longer falling where the calendar said.
The Gregorian rule of 1582 fixes it with a three-part clause: a year divisible by 4 is a leap year, except that a year divisible by 100 is not, except that a year divisible by 400 is. So 1900 was not a leap year and 2000 was. That gives an average year of 365.2425 days, which is within about 27 seconds of the truth — an error of one day in roughly 3,200 years.
What happened to the eleven missing days?
They were deleted on purpose, and different countries deleted them in different centuries.
Pope Gregory XIII's reform of 1582 required removing the accumulated error in one step. In the Catholic countries that adopted it immediately — Spain, Portugal, Italy, Poland — Thursday 4 October 1582 was followed by Friday 15 October 1582. Ten dates simply did not exist.
Protestant and Orthodox countries refused for a long time, on the reasonable grounds that they were not taking calendar instructions from the Pope. Britain and its colonies, including America and India, held out until 1752, by which time the error was eleven days: Wednesday 2 September was followed by Thursday 14 September. Russia waited until 1918 and had to delete thirteen.
The famous riots demanding "give us our eleven days" are largely a myth, based partly on a satirical Hogarth painting. There were real grievances, and they were financial, not superstitious: people wanted to know whether a month's rent was due for a month that was eleven days short, and whether interest accrued on days that did not exist. Britain handled it by shifting the tax year end, which is why the British tax year still ends on the peculiar date of 5 April.
The week and the day
Why does a week have seven days?
It is the only unit of the calendar with no astronomical basis at all. A day is one rotation, a month is roughly one lunar cycle, a year is one orbit. A week is a human invention.
The likeliest origin is Babylonian: seven is the number of moving objects visible in the sky without instruments — the sun, moon, Mercury, Venus, Mars, Jupiter and Saturn — and each was assigned a day. That naming survives almost intact in the Romance languages (lundi is moon-day, mardi is Mars-day, mercredi Mercury) and in English through a Germanic substitution of equivalent gods: Tuesday for Tiw, Wednesday for Woden, Thursday for Thor, Friday for Frigg. Saturday, Sunday and Monday kept the Roman planet directly.
The Jewish sabbath cycle reinforced the same length independently, and the two traditions merged in the Roman empire.
Attempts to change it have all failed. Revolutionary France tried a ten-day week in the 1790s; the Soviet Union tried five-day and six-day weeks in the 1930s to keep factories running continuously. Both were abandoned within a few years, for the same reason: a week that does not synchronise families, markets and rest is socially unusable, however rational it looks on paper.
Why do we work five days and rest two?
The second rest day is a nineteenth-century compromise between two religions, and the five-day week is a twentieth-century business decision.
Sunday was the Christian day of rest and the only day off in most industrial employment. Workers routinely failed to appear on Monday after Sunday drinking — the practice was so common it had a name, Saint Monday. Some British factories from the 1840s began giving Saturday afternoon off instead, on the theory that a half-day of legitimate leisure was cheaper than an unplanned lost Monday.
The full two-day weekend came from a different pressure. A New England mill in 1908 gave Saturday off to accommodate Jewish workers, whose sabbath runs from Friday evening to Saturday evening, so that both they and Christian workers had their day. Henry Ford adopted a five-day, forty-hour week across his plants in 1926 — and his stated reasoning was not generosity. He argued that workers with time and money to spend were the market for the cars, and that a rested workforce was a more productive one. Output did not fall.
The five-day week spread through American industry and was written into US law in 1938. So the shape of your week is: one day from religion, one day from a compromise between two religions, and the length of the working day from an argument about consumer demand.
Why does the day start at midnight?
Because it was the least disruptive choice available, and because the alternatives are worse.
Several traditions have started the day at sunset — the Jewish and Islamic calendars still do, which is why festivals begin the previous evening. Others started at sunrise. Astronomers until 1925 started at noon, so that a single night of observation did not fall across two dates, which is the sort of practical reason that actually decides these things.
Sunset and sunrise both move through the year and vary with latitude, so neither gives a fixed-length day. Noon is stable but falls in the middle of waking hours, which means the date changes while you are working. Midnight is stable, and it puts the discontinuity where the fewest people are awake to be inconvenienced by it.
Why AM and PM, and is 12 PM noon or midnight?
AM is ante meridiem, before midday; PM is post meridiem, after midday. The dividing point is the moment the sun crosses the meridian.
Noon is 12 PM and midnight is 12 AM, which is genuinely illogical — noon is neither before nor after midday, it is midday. The convention exists because the hour beginning at noon is the first hour after midday, so it counts as PM, and the same for midnight and AM.
Because it confuses people, careful writing avoids it. Airlines, hospitals and railways use the 24-hour clock, and legal documents write "11:59 PM" or "12:01 AM" rather than risk a contract that expires at an ambiguous moment.
Time across the world
Why do time zones exist, and who decided where they go?
Because the railway made local time impossible, exactly as described in 2.4.
Every town had set noon by its own sun, so time changed continuously as you travelled — a difference of four minutes for every degree of longitude. Trains made that unworkable, and after Britain standardised internally, the problem went global.
The International Meridian Conference met in Washington in 1884 and settled two things: a single prime meridian, and a system of hourly zones counted from it. Greenwich won the vote with 22 in favour, 1 against (San Domingo) and 2 abstaining (France and Brazil). The reason was commercial, not political — around two thirds of the world's shipping already used charts based on Greenwich, so choosing anything else meant reprinting the world's navigation tables. France used "Paris mean time retarded by nine minutes and twenty-one seconds" for another quarter century rather than say the word Greenwich.
The zones are not tidy, because governments draw them. China spans five geographical hours and uses one time zone for all of it, so the sun rises after 10 AM in winter in Xinjiang. India uses a single half-hour offset, UTC+5:30, chosen as a compromise across a country wide enough to justify two — a permanent minor argument in the north-east, where it is light at four in the morning. Nepal uses UTC+5:45.
What is the difference between GMT and UTC?
GMT is a time zone based on the sun at Greenwich. UTC is a time standard based on atomic clocks, and it is what everything technical actually runs on.
The second was redefined in 1967 in terms of a specific number of oscillations of a caesium atom, because atomic clocks are far more stable than the Earth. The Earth's rotation is not constant — tides slow it gradually, and earthquakes and the movement of mass inside it nudge it either way — so atomic time and solar time drift apart.
Leap seconds were introduced to reconcile them: an extra second is inserted, usually on 30 June or 31 December, to keep UTC within 0.9 seconds of the Earth's actual rotation. Twenty-seven have been added since 1972. They are widely hated by anyone who writes software, because a minute with 61 seconds in it breaks assumptions all over the world's computers, and several large outages have been traced to them. In 2022 the international body responsible agreed to stop adding them by 2035.
Why do computers count time from 1 January 1970?
Because the engineers building Unix in the early 1970s needed an arbitrary zero, picked a recent round date, and it stuck.
Unix time is the number of seconds since that instant, and it is the internal representation of time in an enormous fraction of the world's software. Its advantage is that it is a single integer with no time zones, no months of different lengths and no daylight saving, so comparing two moments is subtraction.
The famous consequence is the Year 2038 problem: a signed 32-bit integer counting seconds overflows on 19 January 2038, and would wrap to a date in 1901. It is the same class of bug as the Year 2000 problem, which was real, was largely fixed by an enormous and expensive effort through the late 1990s, and is now often mocked as a false alarm because the effort worked.
Other calendars
Which calendar does India officially use?
Two. The Gregorian calendar for civil purposes, and the Saka calendar, adopted in 1957, for the official gazette, government communications and All India Radio dates. Saka year 1 corresponds to 78 CE, so the current Gregorian year minus 78 gives roughly the Saka year, and its year begins around 22 March.
Alongside those, dozens of regional calendars remain in daily religious and cultural use — Vikram Samvat in the north, the Bengali San, the Tamil and Malayalam calendars, and others. Most are lunisolar: months follow the moon, and an extra month is inserted periodically to keep the year in step with the seasons, which is why festivals move within a range of a few weeks rather than landing on a fixed Gregorian date.
Why does the Islamic calendar move through the seasons?
Because it is purely lunar, with no correction to the solar year at all. Twelve lunar months come to about 354 days, eleven fewer than a solar year, so every date moves about eleven days earlier each year and completes a full circuit of the seasons in roughly 33 years.
That is a deliberate choice rather than an oversight. It means Ramadan falls in every season over a lifetime, so no region of the world has permanently longer or shorter fasting days than another.
The Hebrew calendar makes the opposite choice: it is lunisolar, and inserts a thirteenth month seven times in every nineteen years, which keeps Passover in spring.
Has anyone tried to fix the calendar?
Repeatedly, and it never sticks.
The French Revolutionary calendar of 1793 had twelve months of exactly thirty days, each divided into three ten-day weeks, with five spare days at the end and beautiful new month names based on the weather. It also decimalised the day into ten hours of a hundred minutes. Decimal time lasted about seventeen months; the calendar itself lasted twelve years before Napoleon abolished it.
The twentieth century produced serious proposals for a perpetual calendar in which every date falls on the same weekday every year, achieved by adding a "blank day" belonging to no week. The League of Nations examined nearly two hundred of them. All failed on the same obstacle: an unbroken seven-day cycle is a religious requirement for several faiths, and a blank day breaks it.
So the calendar you use is a Roman farming year, patched by an emperor, corrected by a pope, adopted at different times in different countries, running on a week borrowed from Babylon and a second defined by a caesium atom. It is not designed. It is sediment.
What comes next
The next Part turns from years to people — the mathematicians, physicists, doctors, engineers and reformers whose names come up in conversation, and what they actually did.