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Add some explanations for gcal

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</nav>
<noscript>You need to enable JavaScript to run this app.</noscript>
<div id="root"></div>
<div class="main">
<h2 id="explanation">Explanation</h2>
<div class="card">
<div class="card-body">
<h4 class="card-title">What is this?</h4>
<p class="lead">This is an implementation of the <a href="https://en.wikipedia.org/wiki/Gregorian_calendar">Gregorian
calendar</a>—which is the default civil calendar in most countries—with conversions to various other
calendars.</p>
<p>Currently, it displays the following conversions for each day, if possible:</p>
<ul>
<li>the <a href="https://en.wikipedia.org/wiki/Julian_calendar">Julian calendar</a> date;</li>
<li>the <a href="https://en.wikipedia.org/wiki/Julian_day">Julian day (JD) number</a>;</li>
<li>the <a href="https://en.wikipedia.org/wiki/Mesoamerican_Long_Count_calendar">Mesoamerican Long Count
calendar</a> date; and
</li>
<li>the <a href="https://en.wikipedia.org/wiki/French_Republican_calendar">French Republican
calendar</a> date.
</li>
</ul>
</div>
</div>
<div class="card">
<div class="card-body">
<h4 class="card-title">How are BCE years handled?</h4>
<p class="lead">The astronomical convention is used, i.e. year 0 is 1 BCE, year -1 is 2 BCE, etc.</p>
<p>Due to the way the common era (a.k.a. Anno Domini) year numbering system works, the year 1 BCE is
followed directly by 1 CE with no year zero. This makes math hard. As such, the astronomical convention
of making 1 BCE year 0 and extending this into the past was used.</p>
</div>
</div>
<div class="card">
<div class="card-body">
<h4 class="card-title">What is the JD (Julian day number) value?</h4>
<p>The integer <a href="https://en.wikipedia.org/wiki/Julian_day">Julian day (JD) number</a> is the
continuous count of days since the beginning of the Julian period, which is very useful in astronomy and
in software for calculating durations without worrying about weird calendar issues. In this respect,
it's very similar to Unix time.</p>
<p>JD 0 is assigned to the date Monday, January 1, 4713 BCE (Julian) or November 24, 4714 BCE (Gregorian).
More specifically, if fractional JDs are considered, then the integer value specifically refers to the
Universal Time noon on that date.</p>
</div>
</div>
<div class="card">
<div class="card-body">
<h4 class="card-title">What is the LC (Mesoamerican Long Count) date?</h4>
<p>The five (or more) numbers separated by dots is the corresponding
<a href="https://en.wikipedia.org/wiki/Mesoamerican_Long_Count_calendar">Mesoamerican Long Count
calendar</a> date. This is commonly known as the &ldquo;Mayan calendar.&rdquo; This calendar is not
available for dates before August 11, 3114 BCE (Gregorian) or September 6, 3114 BCE (Julian).</p>
</div>
</div>
<div class="card">
<div class="card-body">
<h4 class="card-title">What is the FR (French Republican calendar) date?</h4>
<p>The <a href="https://en.wikipedia.org/wiki/French_Republican_calendar">French Republican
calendar</a> was a calendar created and implemented during the French Revolution. It is also frequently
referred to as the <em>French Revolutionary Calendar</em>, but this is a misnomer:
year 1 of the calendar started on 22 September 1792, the day after the
<a href="https://en.wikipedia.org/wiki/Proclamation_of_the_abolition_of_the_monarchy">abolition of the
monarchy</a> and the founding of the <a href="https://en.wikipedia.org/wiki/French_First_Republic">French
First Republic</a>.</p>
<p>Specifically, the variant used here is <a href="https://frcal.qt.ax/">the one I computed from
astronomy</a>.</p>
</div>
</div>
</div>
<footer class="footer">
<div class="container">
<p class="text-muted">Copyright &copy; 2022<%= new Date().getFullYear() > 2022 ? `–${new Date().getFullYear()}`