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<title>B.3.History of Units</title>
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="datetime-units-history"></a>B.3.History of Units</h2></div></div></div>
<p> The Julian Date was invented by the French scholar
Joseph Justus Scaliger (1540-1609)
and probably takes its name from Scaliger's father,
the Italian scholar Julius Caesar Scaliger (1484-1558).
Astronomers have used the Julian period to assign a unique number to
every day since 1 January 4713 BC. This is the so-called Julian Date
(JD). JD 0 designates the 24 hours from noon UTC on 1 January 4713 BC
to noon UTC on 2 January 4713 BC.
</p>
<p> The “<span class="quote">Julian Date</span>” is different from the “<span class="quote">Julian
Calendar</span>”. The Julian calendar
was introduced by Julius Caesar in 45 BC. It was in common use
until the year 1582, when countries started changing to the Gregorian
calendar. In the Julian calendar, the tropical year is
approximated as 365 1/4 days = 365.25 days. This gives an error of
about 1 day in 128 years.
</p>
<p>
The accumulating calendar error prompted
Pope Gregory XIII to reform the calendar in accordance with
instructions from the Council of Trent.
In the Gregorian calendar, the tropical year is approximated as
365 + 97 / 400 days = 365.2425 days. Thus it takes approximately 3300
years for the tropical year to shift one day with respect to the
Gregorian calendar.
</p>
<p> The approximation 365+97/400 is achieved by having 97 leap years
every 400 years, using the following rules:
</p>
<table class="simplelist" border="0" summary="Simple list">
<tr><td> Every year divisible by 4 is a leap year.
</td></tr>
<tr><td> However, every year divisible by 100 is not a leap year.
</td></tr>
<tr><td> However, every year divisible by 400 is a leap year after all.
</td></tr>
</table>
<p>
So, 1700, 1800, 1900, 2100, and 2200 are not leap years. But 1600,
2000, and 2400 are leap years.
By contrast, in the older Julian calendar all years divisible by 4 are leap
years.
</p>
<p> The papal bull of February 1582 decreed that 10 days should be dropped
from October 1582 so that 15 October should follow immediately after
4 October.
This was observed in Italy, Poland, Portugal, and Spain. Other Catholic
countries followed shortly after, but Protestant countries were
reluctant to change, and the Greek orthodox countries didn't change
until the start of the 20th century.
The reform was observed by Great Britain and Dominions (including what is
now the USA) in 1752.
Thus 2 September 1752 was followed by 14 September 1752.
This is why Unix systems have the <code class="command">cal</code> program
produce the following:
</p>
<pre class="screen">$ <strong class="userinput"><code>cal 9 1752</code></strong>
September 1752
S M Tu W Th F S
1 2 14 15 16
17 18 19 20 21 22 23
24 25 26 27 28 29 30</pre>
<p>
</p>
<div class="note" style="margin-left: 0.5in; margin-right: 0.5in;">
<h3 class="title">Note</h3>
<p> The SQL standard states that “<span class="quote">Within the definition of a
‘<span class="quote">datetime literal</span>’, the ‘<span class="quote">datetime
value</span>’s are constrained by the natural rules for dates and
times according to the Gregorian calendar</span>”. Dates between
1752-09-03 and 1752-09-13, although eliminated in some countries
by Papal fiat, conform to “<span class="quote">natural rules</span>” and are
hence valid dates.
</p>
</div>
<p> Different calendars have been developed in various parts of the
world, many predating the Gregorian system.
For example,
the beginnings of the Chinese calendar can be traced back to the 14th
century BC. Legend has it that the Emperor Huangdi invented the
calendar in 2637 BC.
The People's Republic of China uses the Gregorian calendar
for civil purposes. The Chinese calendar is used for determining
festivals.
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