PHP setting of datetime field in CCK through Rules, How to stop continuous updates? Drupal 6. Using Rules and CCK

So, I have a content type that gets automatically generated with Rules. Several CCK fields get populated at creation time, including a datetime field. I want this field to get populated with the current date/time.

For some reason, the Global Token fields don't work, the interface throws an error when I try to use a token instead of a standard formatted date/time stamp.

So, I use advanced PHP code box to set the date/time value.

return array(
0 => array('value' => date("m/d/Y - H:i")),
);

This code works, but when I do it this way it seems that whenever I refresh the node page the date/time stamp in the field is updated to whatever the current time is. This is not the behavior I want. I want the date/time to get inserted at creation time, and have it remain unchanged for perpetuity.

Thoughts?
Thanks to Clive's help, I determined the problem was with the field type. There were several similar seeming Date/Time field types available to use, I was using one called "datetime". I switched to another one called "datestamp" and the datestamp field retained its original value after refreshing. I guess the "datetime" field is designed to always return the current datetime value of now(). Thanks for helping me work through this friend!