Yes, the timecreated field in the user table is a Unix timestamp as are all dates and times internally in the database
If you need to automate this, then you would need to do some coding, to extract and convert that field and push it into the custom profile field.
If you are okay with doing this manually on a periodic basis, you can use the SQL
simple report below I just crufted together. You can use this with the Configurable Reports plugin to give you a properly formatted CSV file that you could then use in the Upload
users tool to fill in the custom profile field.
In this basic example, I have named the custom profile field "yearmonth" formatted as YYYY-MM but it could be changed to be anything you need. of course.
DATE_FORMAT(FROM_UNIXTIME(u.timecreated),'%Y-%m') AS profile_field_yearmonth
FROM prefix_user u
WHERE u.id > 2 AND u.deleted <> 1
ORDER BY u.username
Save this in Config Reports as a CSV file and you can upload
as is to update all existing users. (The custom field itself needs to already exist of course.)
This could help to retro-fit any existing users. But if you need more or less real time updating when new users are created, then you will need some other method, with PHP or web access API. But hopefully this will point you in a useful direction.