I believe it's

update ubbt_USERS set USER_REGISTERED_ON='' where USER_ID=''

but what is the format for the date please?

On my forum 21/07/07 means 21st July 2007

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.

Thanks very much!