Need a better solution than "try-catch" statement to find an intentional error in input arguments

I know there are a lot of handy matlab functions that do what I must program, but I'm not allowed to use them. I need to be able to detect if the user enters the wrong data to the following. One of the test statements is:

y = day_diff(12,3,[3 4],5)

This obviously produces an error and my attempt to use "try-catch" in the code doesn't work. Does anyone have any ideas about how to catch this?

My code is:

function days= day_diff(M1,D1,M2,D2)
if nargin('day_diff') >4
   days=-1;
   return;
end
if D1<1 ||D2  <1
    days = -1;
    return;
end
try a = (M1 - floor(M1)); 
catch days =-1;
    return;
end
try b = (M2 - floor(M2));
catch days = -1;
    return;
end
try c = (D1 - floor(D1));
catch days =-1;
   return;
end
try d = (D2 - floor(D2));
catch days = -1;
    return;
end   
if a ~=0 || b~=0
    days =-1;
    return;
end
if c ~=0 ||d~=0
    days =-1;
    return;
end
if isa(M1,'double') ~=1 || isa(D1,'double') ~=1 || isa(M2,'double') ~=1 || isa(M2,'double')~=1;
days = -1
return;
end
if M2 <1||M2 >12
    days = -1;
    return;
end
if M2 == 2 && D2 >28 || D2<1
    days = -1;
    return;
elseif (M2 == 4 || M2==6||M2==9||M2==11) && (D2>30 ||D2<1)
    days = -1
    return;
elseif D2 >31||D2<1
    days = -1;
    return;
end

if M1 <1||M1 >12
    days = -1;
    return;
end
if M1 == 2 && D1>28 || D1<1   
    days = -1;
    return;
elseif M1 == 4 || M1==6||M1==9||M1==11 && D1>30 ||D1<1
    days = -1
    return;
elseif D1 >31||D1<1
    days = -1;
    return;
end
if M2<M1
    M = M1;
    M1 = M2;
    M2 = M;
    D = D1;
    D1 = D2;
    D2 = D;
end
if M1<M2
    count=M1;
    total=0;
    if count<M2
        if count==4 ||count==6||count==9||count==11
            length=30;
        elseif count==2
            length = 28; 
        else
            length=31;
        end
        total=total+length;
    end
    count = count +1;
    if count <M2
        if count ==4||count==6||count==9||count==11
            length = 30;
        elseif count==2
            length = 28; 
        else
            length = 31;
        end
        total = total +length;
    end
    count = count +1;
    if count <M2
        if count ==4||count==6||count==9||count==11
            length = 30;
        else
            length = 31;
        end
            total = total + length;
    end
    count = count +1;
    if count <M2
        if count ==4||count==6||count==9||count==11
            length = 30;
        else
            length = 31;
        end
            total = total + length;
    end
    count = count +1;
    if count <M2
        if count ==4||count==6||count==9||count==11
            length = 30;
        else
            length = 31;
        end
            total = total + length;
    end
    count = count +1;
    if count <M2
        if count ==4||count==6||count==9||count==11
            length = 30;
         elseif count==2
            length = 28; 
        else
            length = 31;
        end
            total = total + length;
    end
    count = count +1;
    if count <M2
        if count ==4||count==6||count==9||count==11
            length = 30;
        else
            length = 31;
        end
            total = total + length;
    end
    count = count +1;
    if count <M2
        if count ==4||count==6||count==9||count==11
            length = 30;
        elseif count==2
            length = 28; 
        else
            length = 31;
        end
            total = total + length;
    end
    count = count +1;
    if count <M2
        if count ==4||count==6||count==9||count==11
            length = 30;
        elseif count==2
            length = 28; 
        else
            length = 31;
        end
            total = total + length;
    end
    count = count +1;
    if count <M2
        if count ==4||count==6||count==9||count==11
            length = 30;
        elseif count==2
            length = 28; 
        else
            length = 31;
        end
            total = total + length;
    end
    count = count +1;
    if count <M2
        if count ==4||count==6||count==9||count==11
            length = 30;
        else
            length = 31;
        end
            total = total + length;
    end
    count = count +1;
    if count <M2
        if count ==4||count==6||count==9||count==11
            length = 30;
        elseif count==2
            length = 28; 
        else
            length = 31;
        end
            total = total + length;
    end
    count = count +1;
    if count <M2
        if count ==4||count==6||count==9||count==11
            length = 30;
        elseif count==2
            length = 28; 
        else
            length = 31;
        end
            total = total + length;
    end
    count = count +1;
    if count <M2
        if count ==4||count==6||count==9||count==11
            length = 30;
        elseif count==2
            length = 28; 
        else
            length = 31;
        end
            total = total + length;
    end
    count = count +1;
    if count <M2
        if count ==4||count==6||count==9||count==11
            length = 30;
        elseif count==2
            length = 28; 
        else
            length = 31;
        end
            total = total + length;
    end
    count = count +1;
    if count <M2
        if count ==4||count==6||count==9||count==11
            length = 30;
        elseif count==2
            length = 28; 
        else
            length = 31;
        end
            total = total + length;
    end
    count = count +1;
    if count <M2
        if count ==4||count==6||count==9||count==11
            length = 30; 
        elseif count==2
            length = 28; 

          else
              length = 31;
          end
              total = total + length;
      end
  end
  if M1==M2 && D1==D2
      days=0;
      return;
  end
  if M1==M2 && D2>=D1
      days= D2-D1;
  end
  if M1==M2 &&D1>=D2
      days = D1-D2
  end
  if M1+1==M2
     if M1 ==4||M1==6||M1==9||M1==11
         length=30;
     elseif M1==2
              length = 28; 
     else
         length=31;
     end
     days=length-D1+D2;
     return;
  end
  if round(M2+1)==round(M1)
     if M1 ==4||M1==6||M1==9||M1==11
         length=30;
     elseif M1==2
        length = 28; 
     else
         length=31;
     end
     days=length-D2+D1;
  end 
  if round(M1+1)==round(M2)
     if M1 ==4||M1==6||M1==9||M1==11
         length=30;
     elseif M1==2
        length = 28; 
     else
         length=31;
     end
     days=length-D1+D1;
     return;
  end 
  if M2>M1 && M1+1 ~=M2
      days=total-D1+D2;
  end

[Walter Roberson]

There might be a better solution than try/catch, but how do we know whether you are allowed to use them?

[Walter Roberson]

That code is amazingly long for the task. Have you considered using array indexing?

[KSSV]

I would prefer, running the code without try and catch; let the matlab stop and show me the error, so that I can change my code.

[Walter Roberson]

This is a school assignment where the programmer is responsible for detecting all possible errors and handling them smoothly. try/catch can be appropriate in some cases, especially if the programmer is forbidden from using some constructs that might allow the problems to be avoided.