how do I add 365 to each year without messing up the # of days?

Lab4Part1a.java

        if (yearDiff == 0)  // Both dates in same year
        {
            monthsBetween = mm2 - mm1;  // if year is same, day2 must be after day1
            if (monthValid && (monthsBetween == 0))
            {
                daysBetween = dd2 - dd1;
            }
            else if (monthValid)
            {
                daysBetween = daysToEndOfMonth(dd1,mm1);
                daysBetween = daysOfMonth(daysBetween, mm1+1, mm2);
                daysBetween += dd2; 
            }
        }
        else if ((yearDiff > 0) && (monthValid)) // Year2 is after Year 1
        {
            System.out.println();
            daysBetween = daysToEndOfMonth(dd1,mm1);
            daysBetween = daysOfMonth(daysBetween,mm1+1,12+1);    
            daysBetween = daysOfMonth(daysBetween,1,mm2);    
            daysBetween += dd2;    
            daysBetween += yearDiff*365;// *-* add in 365 days for each year inbetween yyyy1 and yyyy2    
        }