<input type=hidden name=add_date value=<?php echo(date("Y-m-d h:i:s")); ?>>
if you are not using php to echo the html tags, but embed the php function into html tags, you should use it like above.
you have mentioned that hidden field could not be added to the database. it should not be the case, in the eyes of php, hidden fields values, text fields values should be the same.
the reason that hidden fields cause you problem is that the value/datatype of your hidden fields are not as you expected. such as if you are expecting a date type value, but when you submit it, value=date("Y-m-d h:i:s") will pass the string date("Y-m-d h:i:s") to a field expected a date, but if you use value=<?php echo(date("Y-m-d h:i:s")); ?>, you will pass a date value.
once again, i am assuming you are embeding the function date() into html tags.
hui