mysql how to NOT update datetime?

I have a table where it has two datetimes, one of them is a 'completed' datetime, therefore, if it is NULL then the user has not completed.

I am trying to do an update, to update their mobile number.

How can i do it so it does not update the datetime?

currently if the datetime is null it gets set to the earliest possible time.

here is my query:

mysql_query("UPDATE action_4_members
                SET `mobile` = '{$mobile}', 
                    `additional_txt` = '{$additional}', 
                    `misc_data` = '{$store_id}' 
              WHERE `id` = '{$member_id}'") or die(mysql_error());

here is my table definition:

CREATE TABLE `action_4_members` (
  `id` int(11) NOT NULL AUTO_INCREMENT,
  `campaign_id` int(11) DEFAULT NULL,
  `mobile` varchar(25) DEFAULT NULL,
  `join_txt` varchar(160) DEFAULT NULL,
  `join_txt_date` datetime DEFAULT NULL,
  `additional_txt` varchar(160) DEFAULT NULL,
  `additional_txt_date` datetime DEFAULT NULL,
  `misc_data` text,
  PRIMARY KEY (`id`)
) ENGINE=MyISAM AUTO_INCREMENT=687 DEFAULT CHARSET=latin1

Copyright License：
Author:「Hailwood」,Reproduced under the CC 4.0 BY-SA copyright license with link to original source & disclaimer.
Link to：https://stackoverflow.com/questions/3553158/mysql-how-to-not-update-datetime