Counting days from two date fields, start counting from 1 instead of 0?

I'm using a PHP function to count days between start date and end date for courses on a website. The function itself works flawless, however I'd like the counting to start from 1 as standard instead of 0.

My code looks like this

function dateDifference($start_date, $end_date) {
    // calulating the difference in timestamps 
    $diff = strtotime($start_date) - strtotime($end_date);
     
    // 1 day = 24 hours 
    // 6 * 60 * 60 = 86400 seconds
    return ceil(abs($diff / 86400));
}

I guess the answer to this question is pretty straight forward however I'm not that wandered in PHP.

Thanks!