struct date{ int y;
                    int m;
                    int d;
 long int calculate_date(/*struct variable */)
     {
      //....
     }
};
void main ()
{/*here how to call function and define struct var*/

}

Erm...is there a question in here somewhere?

i dont know, how to call function then calculate dates . example : 1998,1,12 until this date , all dates

So basically you don't know how to write the program and want us to hold your hand (or do it for you). Sorry, but no go. You'll have to work a little harder than that for your code to count as proof of effort.