This question has been answered by whitebyte. See the first response.
<?php
$profile = $hook->getValue('register.profile');
$extended = $profile->get('extended');
$extended['registerdate'] = date('d-m-y');
$profile->set('extended', $extended);
if ($profile->save()) {
return true;
} else {
$modx->log(modX::LOG_LEVEL_ERROR, '[setRegisterDate]: failed to save user profile!');
return false;
}$extended['registerdate'] = time();
$extended['registerdate'] = time();
<?php
$profile = $hook->getValue('register.profile');
$extended = $profile->get('extended');
$extended['registertimestamp'] = time();
$profile->set('extended', $extended);
if ($profile->save()) {
return true;
} else {
$modx->log(modX::LOG_LEVEL_ERROR, '[setRegisterDate]: failed to save user profile!');
return false;
}