[CentOS] what does the strange sssd log mesages mean?

Hi all,

I have a few centos 6 boxes running sssd, they are talking to OpenLDAP server still runs on centos 5. Today I enabled sssd debugging and found the following lines of messages in logs:

...
(Mon Mar  4 23:11:13 2013) [sssd[be[ldap]]] [get_naming_context] (0x0200): Using value from [namingContexts] as naming context.
(Mon Mar  4 23:11:13 2013) [sssd[be[ldap]]] [sdap_get_server_opts_from_rootdse] (0x0200): No known USN scheme is supported by this server!
(Mon Mar  4 23:11:13 2013) [sssd[be[ldap]]] [sdap_get_server_opts_from_rootdse] (0x0200): Will use modification timestamp as usn!
...

what does the above messages mean: why it uses modification timestamp as usn?  does that have some harmful effects, if time drifts /time zone difference exists in a network? 

Please shed a light. Thanks. 

--Gelen