XSLT Cookbook

Calculating Julian and Absolute Day Numbers from a Specified Date

Problem

You have a date and would like to know the corresponding Julian day number and/or Absolute day number.

Julian Day Versus Julian Dates

Do not make the mistake of confusing the Julian day with the Julian calendar. Joseph J. Scaliger has invented the Julian period so that every year could be associated with a positive number without having to worry about BC/AD. The period starts on January 1, 4713 BC (according to the Julian calendar) and lasts for 7980 years. Astronomers have used the Julian period to assign a unique number to every day since 1 January 4713 BC. This is the Julian day. As I write this chapter, the Julian day is 2,452,376. The curious can use the code in this section to determine the actual date. Another absolute numbering scheme used by N. Dershowitz and E. Reingold in their calendar algorithms begins on January 1, 1 AD. I refer to a number in their system as an Absolute day number. Absolute day 1 corresponds to Julian Day 1,721,426.

Solution

This template will give you the Julian day, given the year, month, and day:

<xsl:template name="date:calculate-julian-day"> <xsl:param name="year"/> <xsl:param name="month"/> <xsl:param name="day"/> <xsl:variable name="a" select="floor((14 - $month) div 12)"/> <xsl:variable name="y" select="$year + 4800 - $a"/> <xsl:variable name="m" select="$month + 12 * $a - 3"/> <xsl:value-of select="$day + floor((153 * $m + 2) div 5) + $y * 365 + floor($y div 4) - ...