You're correct, it is the "USER_REGISTERED_ON" field in the "_USERS" table; the entry is the date in
Unix Time.
There are several online converters that will allow you to convert to/from
Unix Time (one such converter is available on
Online Conversion).
The reason that
Unix Time is used here is that we can convert the value to any format a user could want to display it as by using the built in
php date parameter.