Hi again MCPB,
That would be the same procedure as outlined in this recent thread started by yourself, located here:
- How to change a user join date
You would need the birthday mod to begin with, as phpBB2 does not provide this possibility in a plain vanilla install. Thus making it a mod issue. If the post I referred you to, does not help you, you must continue this in the mod's support thread.
Hope this helps,