Leap years.

int year = 2000;
void setup() {
  size(600, 500);
  frameRate(1);
  fill(#FFFF00);
  textSize(30);
}
void draw() {
  background(0);
  cal_leapyears(50, 250);
  year = year+1;
}
void cal_leapyears(int dotx, int doty) {
  if (year%4==0) {
    text(year+"          This year is leap year", dotx, doty );
  } else if (year%4!=0) {
    text(year+"          This year isn't leap year", dotx, doty);
  } else if (year%4==0 && year%100!=0) {
    text(year+"          This year is leap year", dotx, doty );
  } else if (year%4==0 && year%100==0 && year%400!=0) {
    text(year+"          This year isn't leap year", dotx, doty);
  } else if (year%4==0 && year%100==0 && year%400==0) {
    text(year+"          This year is leap year", dotx, doty );
  }
}