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gcal: explain the Julian calendar and Gregorian transition

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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 J (Julian calendar) date?</h4>
<p class="lead">The <a href="https://en.wikipedia.org/wiki/Julian_calendar">Julian
calendar</a> is the predecessor to the Gregorian calendar, originally introduced by Julius Caesar in 46
BC.</p>
<p>Julius Caesar (with aid of Sosigenes of Alexandria) introduced this calendar to replace the earlier Roman
calendar, which required manual addition of leap months to keep it in synchrony with the seasons.
Unfortunately, it had leap days every four years without question, resulting in the average year
having 365.25 days. However, the actual tropical year is roughly 365.2422 days, resulting in the Julian
calendar gaining a day every 129 years. This means that seasons drift, starting earlier and earlier in
the year.</p>
<p>In 1582, Pope Gregory XIII decided to fix this drift by reducing the number of leap days, motivated by a
desire to keep the March equinox on March 21st, since that value was hardcoded in the calculation for
the date of Easter. To achieve this, he made years divisible by 100 but not by 400 non-leap years,
resulting in 97 leap years every 400 years, resulting in the Gregorian calendar. To bring the equinox
back in alignment, October 5th to October 14th in 1582 were deleted. The result is the Gregorian
calendar.</p>
</div>
</div>
<div class="card">
<div class="card-body">
<h4 class="card-title">How are dates before the adoption of the Gregorian calendar handled?</h4>
<p class="lead">This version uses the proleptic Gregorian calendar for those dates.</p>
<p>Since the Gregorian calendar was adopted at different times in different countries, there's no nice way
of handling the transition. In the future, I might make a version that implements the transition with a
country selector, but this version will stay proleptic forever.</p>
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</div>
<div class="card">
<div class="card-body">
<h4 class="card-title">What is the JD (Julian day number) value?</h4>