use integer variables
use long (especially for JD) as julian date's can exceed the integer limits.
added some naming to the code above to better understand it.
Definition: The Julian date (JD) is a continuous count of days from 1 January 4713 BC.
long JulianDate(int year, int month, int day)
{
long centuries = year/100;
long leaps = centuries/4;
long leapDays = 2  centuries + leaps; // note is negative!!
long yearDays = 365.25 * (year + 4716); // days until 1 jan this year
long monthDays = 30.6001* (month + 1); // days until 1st month
long jd = leapDays + days + monthDays + yearDays 1524.5;
return jd;
}
There are also formulas for calculating back to Gregorian e.g. when will I be 10000 or 20000 days old.

http://aa.usno.navy.mil/faq/docs/JD_Formula.php 

http://quasar.as.utexas.edu/BillInfo/JulianDatesG.html 
coding this is left as an exercise for the reader