Similar situation. I have a "lastchanged" field in my db table. When updating via DataAssisst, after I inserted a hidden field
<input id="HiddenFields_fields" name="HiddenFields_fields" type="hidden" value="<?php echo date("Y-m-d h:m:s"); ?> " />
it works great.
In my Security Assist update page, I can't get it to work.
Missing some concept.