my code works for some inputs, but not others

Hi. My function returns a positive integer scalar that is equal to the difference between the ages of the two children in days. However, my code is working for some inputs but not others.

    function dd = day_diff(month1, day1, month2, day2)
    if (month1 && month2 > 0) || (month1 && month2 <= 12)
        if month1 == 1 && month2 == 1
            if day1 == day2
                total1 = day1;
                total2 = day2;
            elseif day1 ~= day2
                total1 = max(day1,day2);
                total2 = min(day1,day2);
            end
        elseif month1 == 1 && month2 == 2
            total1 = day1;
            total2 = day2 + 31;
        elseif (month1 == 2 && day1 <= 28) && month2 == 1
            total1 = day1 + 31;
            total2 = day2;
        elseif month1 == 1 && month2 == 12
            total1 = day1;
            total2 = day2 + 334;
        elseif month1 == 2 && month2 == 3
            total1 = day1 + 31;
            total2 = day2 + 59;
        elseif month1 == 7 && month2 == 9
            total1 = day1 + 181;
            total2 = day2 + 243;
        else
            dd = -1;
            return
        end
    end
    dd = (max(total1,total2)) - (min(total1,total2));

So I started to test some numbers and it worked for lets say (2, 29,1, 22) for (month1, day 1, month2, day2).When i tested for (1,13,4,30) it did not work.

Answer 1

[Walter Roberson]

The expression

month1 && month2 > 0

Does not mean to test that month1 and month2 are both greater than 0. Instead, it means

(month1 ~= 0) && (month2 > 0)

Likewise

(month1 && month2 <= 12)

means

((month1 ~= 0) && (month2 <= 12))