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.