Convert to/from Julian date numbers

julian.c

int julian(int y, int m, int d) {
    return 367*y -
        (7 * (y + (m+9)/12))/4 -
        (3 * ((y + (m-9)/7)/100 + 1))/4 +
        275*m/9 + d + 1721029;
}

void gregorian(int j, int *year, int *month, int *day) {
    int a = j + 68569;
    int b = 4*a / 146097;
    int c = a - (146097*b + 3) / 4;
    int d = 4000*(c+1) / 1461001;
    int e = c - 1461*d/4 + 31;
    int f = 80*e / 2447;
    int g = f/11;
    int h = e - 2447*f/80;
    int m = f + 2 - 12*g;
    int y = 100 * (b-49) + d + g;

    *year = y; *month = m; *day = h;
}