I would want a time stamp the first time data is entered in Field1. Then if Field1 is changed, it would stay the original stime stamp.You can do this by setting the Default value of your "TimeStamp" field to Now() in Table Design. This will stamp all new records with the current date/time.
http://support.microsoft.com/kb/197592/en-us
http://allenbrowne.com/AppAudit.html