该程序遇到了\0的问题,请帮忙改一下,怎样加'\0'?

function data( var s1:shortstring):shortstring;
 type TIntSet = set of 1..12;
var  s_year,s_month:shortstring;
      present:Tdatetime;
      Year,month,day,year_num:word;
      i:integer;
      Up_term,down_term:Tintset;
begin
   s_year:='';
   showmessage(inttostr(length(s1)));
    for i:=1 to 2 do
   s_year[i]:=s1[i];
   
   Present:= Now;
   DecodeDate(Present, Year, Month, Day);
   if month in Up_term then s_month:='1';
   if month in down_term then s_month:='2';
   showmessage('s_month is'+s_month);
    if (strtoint(s_year)>50 ) then s_year:='19'+s_year
   else   s_year:='20'+s_year;
   showmessage('year is'+s_year);
   showmessage(inttostr(length(s_year)));
   year_num:=abs(year-strtoint(s_year));
   showmessage(inttostr(year_num));
   data:=inttostr(year_num)+s_month;
end;procedure TForm1.Button1Click(Sender: TObject);
var str1,str2:shortstring;begin
str1:='0011110803';
str2:='';
str2:=data(str1);
showmessage(inttostr(length(str2)));
showmessage(str2);
end;
end.
该程序遇到了\0的问题,请帮忙改一下,怎样加'\0'?
?系统有函数能对string1取任意字段吗?怎么用?
先谢谢了!