note 8516 added to mktime

the mktime() function paired with the strftime() function has a nice feature of being able to do date addition and subtraction! <p> If you have a date of 2000-08-01 and you want to know the date if you shift XXX days forward, you can do the following: <p> <code><pre> $yr = "2000"; $mn = "8"; $dy = "1"; $offset = 54; $newday = strftime("%Y-%m-%d", mktime(0,0,0,$mn,$dy+$offset,$yr)); </pre></code> <p> This is great! However, there is one thing you may want to take note of...namely INTEGER limitations. Apparently the mktime() function returns the UNIX time conversion of a date where the values represent seconds since Jan 1, 1970, and are positive or negative depending on whether the date appears before or after 1970. Due to the MAX_INT (LONG?) size, dates can only be represented as far back as 1902 (- MAX) and as far forward as 2037 (+ MAX). <p> Also, the addition of any field that might exceed the max_int limitation itself will cause an invalid calculation and reset the date to Dec 31, 1969 (Unix time of -1). <p> <code><pre> EX: $day_shift = 36524; $stmp = mktime(0,0,0,1,$day_shift,1903); // $stmp == -1 !! // why ? because // 36524 days == 3155673600 secs) </pre></code> <p> In this case, the 3155673600 shift is too large for the mktime() function to compute, so you end up with -1. However, if you do your same shift in smaller increments (say 10,000 days at a time) the calculations will work and you'll be fine ;) http://www.php.net/manual/function.mktime.php