foo.cc: In function 'int main()':
foo.cc:13:14: error: too few arguments to function 'long long int ab(long long int, long long int)'
  d=ac(a)-ab(a);
              ^
foo.cc:4:11: note: declared here
 long long ab(long long a,long long b);//过去的月份天数和 a年 b月 //和判断闰年函数一起用 
           ^~