I was asked this interview question so thought I would post it here to see how other users would answer:

Please write some code which connects to a MySQL database (any host/user/pass), retrieves the current date & time from the database, compares it to the current date & time on the local server (i.e. where the application is running), and reports on the difference. The reporting aspect should be a simple HTML page, so that in theory this script can be put on a web server, set to point to a particular database server, and it would tell us whether the two servers’ times are in sync (or close to being in sync).

This is what I put:

// Connect to database server
$dbhost = 'localhost';
$dbuser = 'xxx';
$dbpass = 'xxx';
$dbname = 'xxx';

$conn = mysql_connect($dbhost, $dbuser, $dbpass) or die (mysql_error());

// Select database
mysql_select_db($dbname) or die(mysql_error());

// Retrieve the current time from the database server
$sql = 'SELECT NOW() AS db_server_time';

// Execute the query
$result = mysql_query($sql) or die(mysql_error());

// Since query has now completed, get the time of the web server
$php_server_time = date("Y-m-d h:m:s");

// Store query results in an array
$row = mysql_fetch_array($result);

// Retrieve time result from the array
$db_server_time = $row['db_server_time'];

echo $db_server_time . '<br />';
echo $php_server_time;

if ($php_server_time != $db_server_time) {
    // Server times are not identical

    echo '<p>Database server and web server are not in sync!</p>';

    // Convert the time stamps into seconds since 01/01/1970
    $php_seconds = strtotime($php_server_time);
    $sql_seconds = strtotime($db_server_time);

    // Subtract smaller number from biggest number to avoid getting a negative result
    if ($php_seconds > $sql_seconds) {
        $time_difference = $php_seconds - $sql_seconds;
    }
    else {
        $time_difference = $sql_seconds - $php_seconds;
    } 

    // convert the time difference in seconds to a formatted string displaying hours, minutes and seconds
    $nice_time_difference = gmdate("H:i:s", $time_difference);

    echo '<p>Time difference between the servers is ' . $nice_time_difference;
}
else {
    // Timestamps are exactly the same
    echo '<p>Database server and web server are in sync with each other!</p>';
}

Yes, I know that I have used the deprecated mysql_* functions but that aside, how would you have answered, i.e. what changes would you make and why? Are there any factors I have omitted which I should take into consideration?

The interesting thing is that my results always seem to be an exact number of minutes apart when executed on my hosting account:

2012-12-06 11:47:07

2012-12-06 11:12:07