Editing D'ni time conversion (section)

==Human Timekeeping==

Before delving into the D'ni timekeeping system, some background in human timekeeping is required.

The calendar of primary interest in this discussion is the Gregorian calendar, which was proclaimed by Pope Gregory in 1582 and is now universally used in western countries and in international commerce. A great deal of in-depth information on this calendar, as well as other human calendars, is available from Web articles such as [http://www.fact-index.com/g/gr/gregorian_calendar.html Fact Index: Gregorian Calendar] and the [http://www.fourmilab.com/documents/calendar/ Fourmilab Calendar Converter]. However, only a few basic facts are important for the purposes of this article.

The Gregorian calendar is based on an average solar year of 365.2425 days, divided into common years of 365 days and leap years of 366. The calendar can be extended to dates prior to its invention by following the calendars rules for determining leap years. This extension is known as the "proleptic" Gregorian calendar.

In common usage, the calendar is divided into two eras: an "AD" era that counts years forward from the year 1 AD, and a "BC" era that counts years backward from the year 1 BC, which is the year prior to 1 AD. In academic practice, the AD era is designated with the abbreviation "CE" (Common Era), and the BC era is designated "BCE" (Before Common Era). Among astronomers, however, all years are treated as part of the common era, and the year 1 BCE is considered the year 0 CE, while earlier years are given negative numbers. Thus, -1 CE = 2 BCE, -2 CE = 3 BCE, etc. This is important because it helps to simplify the calculation of the difference between two dates, which is a key part of the process of calendar conversion.

Because astronomers often need to do arithmetic with dates, they have invented another concept that is useful to our purpose. This is the idea of representing a date as a Julian day number, which is the number of days and fractions that have elapsed since noon on January 1, 4713 BCE. By storing dates in this form, the difference between two dates can easily be determined by simply subtracting their Julian day numbers. Julian day numbers are well suited to computer storage, and are easily converted to other calendar systems, including the Cavernian calendar.

Many programming languages and software packages, such as Microsoft Excel, use a similar system of representing date/time values as "serial numbers". However, most of those serial-number systems are based on a relatively recent date, such as January 1, 1900 CE, and many cannot handle dates prior to their base. The Julian-day system is able to handle dates over a range of more than 10 million years when using double-precision values, so it is easily able to account for any date in the 10,000-year history of the D'ni civilization.

One final concept of importance is that of the "mean solar tropical year", which is fully described by the Web article [http://www.fact-index.com/t/tr/tropical_year.html Fact Index: Tropical Year]. The Gregorian average year length of 365.2425 days is a slight over-estimate of the time between vernal equinoxes, which is only one possible measure of the time required for the Earth to return to the same position along the path of its orbit (the ecliptic). A different estimate will result from choosing a different point on the ecliptic – other than the vernal equinox point – from which to measure. Astronomers have defined the mean solar tropical year as the average length of a year over all points on the ecliptic, which is about 365.2422 days. More precise estimates are available, but these vary depending on when the measurement was performed, and the time units (atomic time, universal time, etc.) used to perform the calculation.