time difference between forum database and time()

[bosal]

Hello.

I want to make simple script in PHP, that will check, if latest thread in specified board is older than 7 days, and if so, random thread from different board will be moved to that board. I just have small problem about it, because i don't what time difference is between time() and dateline in database.

Here's what i'm doing:

$tsql = mysql_query("SELECT threadid,title,dateline FROM thread WHERE forumid=$poll_forum_id and open=1 ORDER BY dateline DESC LIMIT 1");
$temat=mysql_fetch_array($tsql);

$timediff=2*60*60-7; // it looks like around 2 hours, but not exactly, i have no idea what this value should be.

if ($temat['dateline']+$timediff<time()-7*24*60*60) // is thread older than 7 days
{
$tsql = mysql_query("SELECT threadid FROM thread WHERE forumid=158 and open=1 ORDER BY rand() LIMIT 1");
if (mysql_num_rows($tsql)>0)
{
$th=mysql_fetch_array($tsql);
mysql_query('UPDATE `thread` SET (
forumid='.$poll_forum_id.',
dateline='.now()+$timediff .') where threadid='.$th['threadid']);
}
}

For me this difference looks like something around 2 hours, but not exactly, few secounds less. How can i get this difference and be sure, than when time will be changed from summer to winter it will still work fine?