Blog

The Julian date (JD) is the interval of time in days and fractions of a day, since 4713 B.C. January 1, Greenwich noon

The Julian day system was introduced by astronomers to provide a single system of dates that could be used when working with different calendars and to unify different historical chronologies. Apart from the choice of the zero point and name, this Julian day and Julian date are not directly related to the Julian calendar, although it is possible to convert any date from one calendar to the other.

So, I found a couple of methods to calculate the Julian date and here they are:

1) The wikipedia way:

int a = ( 14 - month )/12;
int y = <year> + 4800 - a;
int m = <month> + 12 * a - 3;
double jdn = <day> +
     ( ( ( 153 * m ) + 2 ) / 5 ) + 
     ( 365 * y ) +
     ( y/4 ) -
     ( y/100 ) +
     ( y/400 ) -
     32045;

2) Another way:

yy = (int)( y - Math.floor( ( 12 - m ) / 10 ) );
mm = m + 9;
if( mm >= 12 ) mm -= 12;

k1 = (int)(( Math.floor( 365.25 * ( yy + 4712 ) ) ) );
k2 = (int)(( Math.floor( 30.6 * mm + 0.5 ) ) );
k3 = (int)(( Math.floor( Math.floor( ( yy / 100 ) + 49 ) * 0.75 ) - 38 ) );

jd = k1 + k2 + d + 59;
if ( jd > 2299160) jd -= k3;  // for gregorian caledar

Enjoy!