How to determine a date when user became a member of sharepoint group?

Hi,

I would like to ask , how can I programatically get a date when user became a member of certain group.  Is there a simple way how to get this date from SPUser or SPGroup classes (e.g  is this information stored in m_arrMembersData object array of SPGroup class? - if yes, is there a wrappper/class capable to read this dates from m_arrMembersData array in strongly typed way?) or do I have to use SPAudit class?   

Thanks
Peter