File MSNONMBR has a field (NMDTOPN) that is the date the non-member record was created.  Although that date doesn’t appear on the non-member maintenance screen (Tool #997 "Work With Non-Member Database"), you can Query it.  It is currently not used for specific processing, and your credit union might have old records (brought in as part of your conversion, for example) that do not have an open date on them.  But new records created whenever you add non-members in CU*BASE will record a date in this field.