Perform node validation before a node is created or updated.
This hook is invoked from node_validate(), after a user has has finished editing the node and is previewing or submitting it. It is invoked at the end of all the standard validation steps, and after the type-specific hook_validate() is invoked.
To indicate a validation error, use form_set_error().
Note: Changes made to the $node object within your hook implementation will have no effect. The preferred method to change a node's content is to use hook_node_presave() instead. If it is really necessary to change the node at the validate stage, you can use form_set_value().
$node: The node being validated.
$form: The form being used to edit the node.
$form_state: The form state array.
Note: this list is generated by pattern matching, so it may include some functions that are not actually implementations of this hook.
function hook_node_validate($node, $form, &$form_state) {
if (isset($node->end) && isset($node->start)) {
if ($node->start > $node->end) {
form_set_error('time', t('An event may not end before it starts.'));
}
}
}