In order to get an accurate 24 hr deactivation, you will need to change the joindate
sql field to type "datetime" which will also hold the time. Then use this code:
// In the following statement, $JoinDate holds the sql `joindate` field
$timeDiff = time() - strtotime($JoinDate);
if($timDiff/3600 > 24) {
// code to be executed if it's past the 24 hrs period
}