Date formula hurts my head, have you seen it before and how does it work?

Projects Programming
3

Not sure I've seen that exact formula. I use the function below, which does allow for leap years. Looks like it has at least some similarities.

//Return ordinal day of year for the given time_t
int ordinalDate(time_t t)
{
    int m = month(t);
    int d = day(t);
    
    if (m == 1)
        return d;
    else if (m == 2)
        return d + 31;
    else {
        int n = floor(30.6 * (m + 1)) + d - 122;
        return n + (isLeap(t) ? 60 : 59);
    }
}

//Leap years are those divisible by 4, but not those divisible by 100,
//except that those divisble by 400 *are* leap years.
//See Kernighan & Ritchie, 2nd edition, section 2.5.
boolean isLeap(time_t t)
{
    int y = year(t);
    return (y % 4 == 0 && y % 100 != 0) || y % 400 == 0;
}