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Time Zones: Leap Year and Leap Second Explained

Daylight Savings Time - When we change our clocks. Spring forward, Fall back

How time zones confused the world
Time zones are as much about politics, diplomacy and rebellion as they are about ... the time. In the beginning it was the trains that confused everyone. As railway systems grew, so did the need for a common time. In the US, each town set its own clock, leading to massive confusion among passengers and train operators. Train operators whittled it down to 100 time zones, before eventually adhering to four zones across the US. India's single time zone is also a legacy of Britain's railway construction. China is another vast country which shares a single time zone. The whole country keeps to Beijing time. Not content with our time on Earth, mankind has also attempted to impose the structure of time on the Moon with a measure of time called a lunation - 29.530589 earth days, or the period it takes to go around Earth.


Folklore associated with "leap year" by  Michael McKernan PhD

In Anglophone societes, especially the USA, leap years have had a gender-role inversion custom, which in present-day USA can be found transformed into Sadie Hawkins, TWIRP or Tolo (just in Washington State) high school events (and also some practices outside the high school context).  Since the 1840s, most of the events/practices of this inversion phenomenon in the USA have been strongly connected with social dancing, even though the European origins of the inversion do not seem to have had this dance connection.
Sadie Hawkins events, originated by Al Capp (Alfred Caplin) in his Li'l Abner comic strip in the 1930s as a race (not a dance), quickly came to be social dance events ('Sadie Hawkins Dances'), despite Capp's attempt to control his 'creation'--which clearly was derived from leap year inversions, but quite a bit altered by Capp's personal 'twist' on gender relations.

In my research on leap year cultural practices, I came across very scanty indications of some other (non-inversion, non-birthday) folkloric items, none of which seem to have any currency, or much historical documentation. For instance, there apparently were some beliefs concerning leap years
being unlucky, especially in connection with some farm/orchard crops.
As a USA calendrical phenomenon, leap year/leap day is rather unique in not occuring annually, so attaching cultural practices to the year (somewhat unique as well, except perhaps in China?) or the day (02/29) causes some problems.  About the only significant analogy in USA culture is the (presidential) election cycle; internationally, the Olympic Games have a 4-year cycle, etc.  These, of course, are highly institutionalized, whereas leap years (even the calendrical event per se) are not managed/mandated to the same degree, no matter how intrinsic to our present calendar leap years are.  So (almost) every four years, we have to remind ourselves that a leap year is taking place.  Editors and media producers then hand out assignments to their reportorial staff--usually a
junior employee--to produce something on 'leap year.'  This appears to have resulted in quite a bit of 'fakelore.'  (For example, I've seen a number of news stories make the claim that the annual Sadie Hawkins Day' ''occurs on February 29''!!!) So try not to take all that too seriously.


Changes in 2007
The United States has planned a change to its DST observance beginning in 2007. The Energy Policy Act of 2005 mandates that DST will start o March 11 and stop on November 4th. These dates are different from previous DST start and stop dates. In 2006, the dates were the first Sunday in April (April 2, 2006) and the last Sunday in October (October 29, 2006). Some countries are still evaluating whether they will adopt the new rules for themselves. You should anticipate more changes in DST and time zone rules for countries that typically align with U.S. DST rules.
If you are concerned about application failures that may result from these DST changes, you should update your Java Runtime Environment.

Summary of the Issues:
Leap second considered harmful R. A. Nelson et al,
"The Leap Second: Its History and Possible Future"
Metrologia volume 38 (2001), pages 509-529
Since 1966, most of the United States has observed Daylight Saving Time from 2:00 a.m. on the first Sunday of April to 2:00 a.m. on the last Sunday of October.
Beginning in 2007, most of the U.S. will begin Daylight Saving Time at 2:00 a.m. on the second Sunday in March and revert to standard time on the first Sunday in November. In the U.S., each time zone switches at a different time.
In the European Union, Summer Time begins and ends at 1:00 a.m. Universal Time (Greenwich Mean Time). It begins the last Sunday in March and ends the last Sunday in October. In the EU, all time zones change at the same moment.

From the WSJ online:
"Word of the U.S. proposal, made secretly to a  United Nations body, began leaking to scientists earlier this month. The plan would simplify the world's timekeeping by making each day last  exactly 24 hours."

Leap Second


Leap second considered harmful Bob Frankston January 3, 2006
This is useful because it strongly affirms my claim that the leap second is a bad solution to a non-problem.
The primary reason for introducing the concept of the leap second  was to meet the requirement of celestial navigation to keep the  difference between solar time and atomic time small. However, the motivation for the leap second has diminished because of the wide  availability of satellite navigation systems, such as GPS, while the operational complexities of maintaining precise timekeeping systems have made the insertion of leap second adjustments increasingly difficult and costly.
In option #7 they note It is thus unlikely that the growing  difference between clock time and levels of daylight would be noticeable for the foreseeable future. Perhaps they are trying to fill up 21 pages in order to get published but it is quite clear that they are saying that the leap second only exists because of modern precision the very same tools that give us the precision also give us the means for dealing with it as a correction factor.
Too bad they can't come right out and say get rid of it but they come quite close.I'll admit that Im bad at this kind of stuff because if the high order bit is wrong I dont spend time on the low order bits. IN this case the high order bit is to introduce perverseness in basic computation rather than keeping it confined to a correction factor. I can only wonder why we ever allowed the leap second to happen in the first place.
It would be nice to get back on track and declare SUT (Simple Universal Time) based on time as if the leap second were never introduced and let the use of UTC be deprecated. That would add another set of calls to the time libraries but the code would be the same as the current UTC code since weve vary rarely implemented real UTC but it would accommodate programs that actually did.

There's a nice summary of the issues at which can also be found (free!)

Leap Year 2000 Explained

Date: Wed, 24 Mar 1999

The year 2000 leap year.

Some years ago, a Digital employee called Stan Rabinowitz was called upon to answer a customer's complaint that VMS "incorrectly" handled the year 2000 as a leap year. His extremely thorough response has since become quite famous in VMS circles, as it is probably the only SPR response to mention Sosigenes, Regiomontanus and the Council of Trent. The following is the original draft of this response, although how it left Digital is a matter of some conjecture. Digital management made him remove the references to radio station WWV, atomic clocks and VMS V4 before sending it to the customer.

It has been reformatted slightly for easy viewing.

SPR Answer Form
SPR NO. 11-60903
System Version Product Version Component
Software VAX/VMS V3.2 VAX/VMS V3.2 Run-Time Library

The LIB$DAY Run-Time Library service "incorrectly" assumes the year 2000 is a leap year.

Thank you for your forward-looking SPR.

Various system services, such as SYS$ASCTIM assume that the year 2000 will be a leap year. Although one can never be sure of what will happen at some future time, there is strong historical precedent for presuming that the present Gregorian calendar will still be in effect by the year 2000. Since we also hope that VMS will still be around by then, we have chosen to adhere to these precedents. The purpose of a calendar is to reckon time in advance, to show how many days have to elapse until a certain event takes place in the future, such as the harvest or the release of VMS V4. The earliest calendars, naturally, were crude and tended to be based upon the seasons or the lunar cycle.

The calendar of the Assyrians, for example, was based upon the phases of the moon. They knew that a lunation (the time from one full moon to the next) was 29 1/2 days long, so their lunar year had a duration of 354 days. This fell short of the solar year by about 11 days. (The exact time for the solar year is approximately 365 days, 5 hours, 48 minutes, and 46 seconds.) After 3 years, such a lunar calendar would be off by a whole month, so the Assyrians added an extra month from time to time to keep their calendar in synchronization with the seasons.

The best approximation that was possible in antiquity was a 19-year period, with 7 of these 19 years having 13 months (leap months). This scheme was adopted as the basis for the religious calendar used by the Jews. (The Arabs also used this calendar until Mohammed forbade shifting from 12 months to 13 months.)

When Rome emerged as a world power, the difficulties of making a calendar were well known, but the Romans complicated their lives because of their superstition that even numbers were unlucky. Hence
their months were 29 or 31 days long, with the exception of February, which had 28 days. Every second year, the Roman calendar included an extra month called Mercedonius of 22 or 23 days to keep up with the solar year.

Even this algorithm was very poor, so that in 45 BC, Caesar, advised by the astronomer Sosigenes, ordered a sweeping reform. By imperial decree, one year was made 445 days long to bring the calendar back in step with the seasons. The new calendar, similar to the one we now use, was called the Julian calendar (named after Julius Caesar). Its months were 30 or 31 days in length and every fourth year was made a leap year (having 366 days). Caesar also decreed that the year would start with the first of January, not the vernal equinox in late March.

Caesar's year was 11 1/2 minutes short of the calculations recommended by Sosigenes and eventually the date of the vernal equinox began to drift. Roger Bacon became alarmed and sent a note to Pope Clement IV, who apparently was not impressed. Pope Sixtus IV later became convinced that another reform was needed and called the German astronomer, Regiomontanus, to Rome to advise him. Unfortunately, Regiomontanus died of the plague shortly thereafter and the plans died as well.

In 1545, the Council of Trent authorized Pope Gregory XIII to reform the calendar once more. Most of the mathematical work was done by Father Christopher Clavius, S.J. The immediate correction that was adopted was that Thursday, October 4, 1582 was to be the last day of the Julian calendar. The next day was Friday, with the date of October 15.

For long range accuracy, a formula suggested by the Vatican librarian Aloysius Giglio was adopted. It said that every fourth year is a leap year except for century years that are not divisible by 400. Thus 1700, 1800 and 1900 would not be leap years, but 2000 would be a leap year since 2000 is divisible by 400. This rule eliminates 3 leap years every 4 centuries, making the calendar sufficiently correct for most ordinary purposes. This calendar is known as the Gregorian calendar and is the one that we now use today. (It is interesting to note that in 1582, all the Protestant princes ignored the papal decreeand so many countries continued to use the Julian calendar until either 1698 or 1752. In Russia, it needed the revolution to introduce the Gregorian calendar in 1918.)

This explains why VMS chooses to treat the year 2000 as a leap year. Despite the great accuracy of the Gregorian calendar, it still falls behind very slightly every few years. If you are very concerned about this problem, we suggest that you tune in short wave radio station WWV, which broadcasts official time signals for use in the United States. About once every 3 years, they declare a leap second at which time you should be careful to adjust your system clock. If you have trouble picking up their signals, we suggest you purchase an atomic clock (not manufactured by Digital and not a VAX option at this time).


When Does Daylight Time Begin and End?

As of 2005 daylight time begins "on the first Sunday in April and ends on the last Sunday in October. ... These dates were recently modified with the passage of the Energy Policy Act of 2005. ...
Starting in March 2007, daylight time will begin on the second Sunday in March and end on the first Sunday in November." Includes a brief history of daylight saving time in the U.S. U.S. Naval Observatory, Astronomical Applications Department.

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