diff --git a/phpBB/install/database_update.php b/phpBB/install/database_update.php
index 360e12e0b6..a3a51324a9 100644
--- a/phpBB/install/database_update.php
+++ b/phpBB/install/database_update.php
@@ -2613,25 +2613,24 @@ function change_database_data(&$no_updates, $version)
 			// If the column exists, we did not yet update the users timezone
 			if ($db_tools->sql_column_exists(USERS_TABLE, 'user_dst'))
 			{
-				// Update timezones
-				// user_dst is 0 if not in dst and 1 if in dst;
-				// this happens to be exactly the correction that should be added to the timezone offset
-				// to obtain dst offset.
-				// Parenthesize here because we operate on this value later.
-				$active_offset = '(user_timezone + user_dst)';
+				// Update user timezones
+				$sql = 'SELECT user_dst, user_timezone
+					FROM ' . USERS_TABLE . '
+					GROUP BY user_timezone, user_dst';
+				$result = $db->sql_query($sql);
 
-				// Now we have a tricky problem of forcing the plus sign into the expression.
-				// Build it via a conditional since there cannot be a portable printf equivalent in databases.
-				// Note that active offset is not an absolute value here - it is an expression that will
-				// be evaluated by the database during query execution.
-				// We don't print - (minus) here because it will come from active offset.
-				$sign = $db->sql_conditional("$active_offset < 0", '', '+');
+				while ($row = $db->sql_fetchrow($result))
+				{
+					$sql = 'UPDATE ' . USERS_TABLE . "
+						SET user_timezone = '" . $db->sql_escape(_convert_phpbb30_timezone($row['user_timezone'], $row['user_dst'])) . "'
+						WHERE user_timezone = '" . $db->sql_escape($row['user_timezone']) . "'
+							AND user_dst = " . (int) $row['user_dst'];
+					_sql($sql, $errored, $error_ary);
+				}
+				$db->sql_freeresult($result);
 
-				// Use database-specific escaping because strings are quoted differently by different databases.
-				$new_value = $db->sql_concatenate($db->sql_escape('GMT'), $sign, $active_offset);
-				$sql = 'UPDATE ' . USERS_TABLE . '
-					SET user_timezone = ' . $new_value;
-				_sql($sql, $errored, $error_ary);
+				// Update board default timezone
+				set_config('board_timezone', _convert_phpbb30_timezone($config['board_timezone'], $config['board_dst']));
 
 				// After we have calculated the timezones we can delete user_dst column from user table.
 				$db_tools->sql_column_remove(USERS_TABLE, 'user_dst');