To use a the current date/time in your JModel entended class, do the following.
In the store() method after the $row->bind() method, add:
//Get a JDate object
$createdate = &JFactory::getDate();
//Create a date in MySQL's datetime format
$row->created = $createdate->toMySQL();
//PS: make sure your table have a created field :)
Joomla!'s JDate also allows you to set custom Unix dates simply by passing in the date to JFactory::getDate() method. E.g.
$createdate =& JFactory::getDate(time());
Note that you wil override the date everytime you save to the database. See if you can modify the model to set the create date only on new saves. :)
See Joomla!'s API documentation for more on what JDate can do.