1. CS240A: A Short History of Time
Carlo Zaniolo
Department of Computer Science
University of California, Los Angeles

2. Calendars
Thirty days hath September, April, June, and November; All the rest have thirty-one Excepting February alone: Which hath but twenty-eight, in fine, Till leap year gives it twenty-nine.
Blue moon

3. Lunar Calendars versus Solar ones
Keeping track of time was a major challenge for early civilizations--an issue involving science, religion and magic.
Because of the difficulty of synchronizing days, lunar cycles (between 29 and 30 days) and solar years.
a 12-month lunar year comes about days short of solar year that has between 365 and 366 days.
The Roman calendar adopted in the western world is a solar calendar consisting of 12 months and 365 days, with a leap year of 366 days---every four year.
This is called Julian Calendar because it was set by Julius Caesar in 45 B.C. The month of July was also named after him.

4. Islamic calendar or Hijri Qamari calendar
Islamic calendar or Hijri Qamari calendar. A feature of the Islamic calendar is that a year is always 12 months, so the months are not linked with the seasons and drift each solar year by 11 to 12 days. It comes back to the position it had in relation to the solar year approximately every 33 Islamic years. It is used mainly for religious purposes, and in Saudi Arabia it is the official calendar. In other systems, a lunar calendar may include extra months added that synchronize it with the solar calendar. A lunisolar calendar is a calendar in many cultures whose date indicates both the moon phase and the time of the solar year.

5. The Gregorian Calendar
The Julian calendar had too many leap years.
Thus, in 1582 AD, Pope Gregory XIII (helped by astronomer Christopher Clavius) introduced the Gregorian calendar that modifies the Julian as follows:
Years ending with 00 must also be divisible by 400 in order to have 29 days in February
Because of Gregory's improved leap year rule, the Western calendar is keeping pace with the seasons (same as Earth’s revolution around sun)
But adoption was neither simple nor quick, since
10 days were skipped in October 1582: Thursday, October 4, 1582-Julian was followed by Friday, October 15-Gregorian.
Protestant and orthodox countries move to the new calendar much later.

6. Slow Adoption
Great Britain & colonies adopted the Gregorian calendar in by then, eleven days had to be eliminated from the Julian calendar.
(At the same time, they changed the beginning of their legal year from March 25 to January 1.)
This is the calendar implemented in DBMSs—modulo dependencies due to system & location [Paul Eggert Posix time guru scripsit]
In Russia the switch was put into effect on January 1, after the October revolution that occurred on October 25, 1917 (actually, on November 7, 1917!)
-- (leap secondsBut will be discussed later.)

7. The Seven-Day Week
Seven days in the Bible. Seven days also used in Egypt, Persia, India, Tibet, Burma and many other sites of civilizations.
This is because the weekly cycle was set after the seven `planets’ that in ancient times were seen to circle the Earth: Sun, Moon, Mars, Mercury, Jupiter, Venus, Saturn
Romance languages have keep those names, except for Sunday which was given a Christian name.
The days of the week in English, are denoted with Anglo-Saxon names for those same seven heavenly bodies. Likewise in all Germanic languages (ditto for the names of their Gods--Tiu, Woden, Thor, Frey).

8. Keeping Time
The time units were set very early: Division of the day in 12 hours traces back to Egyptians.
Subdividing hours and minutes into 60 was probably done to facilitate computations
But the ancient instruments for measuring time were inaccurate.
Sundials (shadow clocks): daytime clocks for sunny days. However, between different seasons, there can be a shift up to 15 minutes
Water clocks and clepsydras for nights and rainy days.
Keeping accurate time over several days was impossible.

9. Modern Times
In 1656, Christiaan Huygens, a Dutch scientist, made the first pendulum clock. The pendulum has a “natural” period of oscillation (Galileo Galilei). The error was reduced to less than 1 minute a day.
Spring-powered clocks were invented by Peter Henlein at the beginning of 1500.
Accurate spring clocks became critical for oceanic navigation (determining longitude). In 1761 John Harrison won the British government's $2,000,000 prize (in today's money) for a marine chronometer with a spring and balance wheel escapement that was accurate to one-fifth of a second a day.
Today we have: Quartz clocks, atomic clocks (based on cesium), and time-servers on the web.
The Coordinated Universal Time (UTC) uses atomic time. But every year, the international commission can prescribe an adjustment of one or two seconds as needed to compensate for slowing or earth`s rotation (leap seconds).

10. Time Zones Also see http://physics.nist.gov/GenInt/Time/time.html
The U.S.A. was divided in four time zones in Until then, most cities relied upon their own local “sun time,” which changes by approximately one minute for every 12 miles traveled east or west. This became a major cause of train accidents—thus the time zones.
The World's Time Zones were established in 1884.
Greenwich, England, was designated the center of the zones. The international dateline was drawn to generally follow the 180 meridian in the Pacific Ocean
Because of some countries, islands or states do not want to be divided into several zones, the zones' boundaries tend to wander considerably from straight north-south lines.
Summer time: not all states/countries switch---those that do, often switch on different days.

11. On the web Gregorian Calendar (Introduced in 1582)
Gregory XIII
Christopher Clavius
Inter Gravissimas
A slow transition (1752)
Eastern Orthodox (1923)
Longitude and the movie on John Harrison