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.