The date is not stored in the database. But it might be possible to get the date when a client got created by looking on the creation date of the client folder in /var/www/clients/
Ok. Might be bthat I had a older version here that I looked at. The field contains a timestamp in seconds counted from 1. january 1970, thats the way dates are counted in Linux. You can calculate a human readable format of a timestamp e.g. with the date function in php.