Calculating the days from J2000 ------------------------------- The tables below can be used to calculate the number of days and the fraction of a day since the epoch J2000. If you need the number of Julian centuaries, then just divide the 'day number' by 36525. Table A | Table B Days to beginning of | Days since J2000 to month | beginning of each year | Month Normal Leap | Year Days | Year Days year year | | | | Jan 0 0 | 1998 -731.5 | 2010 3651.5 Feb 31 31 | 1999 -366.5 | 2011 4016.5 Mar 59 60 | 2000 -1.5 | 2012 4381.5 Apr 90 91 | 2001 364.5 | 2013 4747.5 May 120 121 | 2002 729.5 | 2014 5112.5 Jun 151 152 | 2003 1094.5 | 2015 5477.5 Jul 181 182 | 2004 1459.5 | 2016 5842.5 Aug 212 213 | 2005 1825.5 | 2017 6208.5 Sep 243 244 | 2006 2190.5 | 2018 6573.5 Oct 273 274 | 2007 2555.5 | 2019 6938.5 Nov 304 305 | 2008 2920.5 | 2020 7303.5 Dec 334 335 | 2009 3286.5 | 2021 7669.5 Worked Example To find the number of days from J2000.0 for 1999 May 4th, at 3.5 hours UT 1. find from table A the number of days to the beginning of May from the start of the year, here 120 days 4. write down the day number within the month, here 4 above 5. find from table B the days since J2000.0 to the beginning of the year, here -366.5 6. add these three numbers, and add 3.5/24 as the day fraction For the date above; 120 + 4 + -366.5 + 3.5/24 = -242.3542 days from J2000.0 7. to find the number of Julian centuries t, divide by 36525, t = -242.3542 / 36525 = - 0.0066353 8. to find the Julian day number, just add 2451545 days to your answer to step 6.