Why does adding and subtracting integers and longs get weird?

whymathbad.c

//
//  main.c
//  ch10
//
//  Created by john salyer on 2/12/13.
//  Copyright (c) 2013 john salyer. All rights reserved.
//

#include <stdio.h>
#include <time.h>

int main(int argc, const char * argv[])
{

    //what is now?
    
    long secondsSince1970 = time(NULL);
    
    struct tm now = *localtime(&secondsSince1970);  //the number of seconds since jan 1 1970
    
    //localtime_r(&secondsSince1970, &now);
    
    printf("The date now is %d-%d-%d\n", now.tm_mon+1, now.tm_mday, now.tm_year+1900);
    
    
    printf("days since Jan 1: %d \n",now.tm_yday+1);
    
    //what is 4M seconds from now?
    
    long fourmillion = 4000000;
    
    long plusfourmillion = secondsSince1970 + fourmillion;
    
    long then = time(&plusfourmillion);
    
    struct tm future;// = *localtime(&secondsSince1970);
    
    localtime_r(&then, &future);
    
    printf("The date in 4 million seconds will be %d-%d-%d\n", future.tm_mon+1, future.tm_mday, future.tm_year+1900);
    
    //it should be something like 46.296296296 days!!!
    
    printf("days since Jan 1: %d \n",future.tm_yday+1);
    
    //why is it 47?
    printf("What's %d - %d = %d\n",future.tm_yday+1, now.tm_yday+1, future.tm_yday - now.tm_yday );
    
    
    //shouldn't be the number of seconds that we're calculating?
    printf("Tell us the difference between NOW %li and THEN %li equals %li", secondsSince1970, then, secondsSince1970 - then);
    // -9 !?!?!  WTF!?
    
    
    return 0;
}