[dbo].[SYSUSERLOG] and Log out dates

Hi all

I am wanting to understand something about how Dynamics works when a user logs out.  I have found a table called [dbo].[SYSUSERLOG] which shows a CREATEDDATETIME which I am assuming is login time, and a LOGOUTDATETIME which I assume is when a user logs out.

The problem is I often see timestamps in the logout of 1900-01-01 00:00:00.000 - I am wondering why does AX log things as this timestamp and what can I do to alleviate the problem?

Thanks