Hi Russ,
The UME API method getLastSuccessfulLogonDate is deprecated, because it does not return valid information, unless you configure your system to write last successful logon date. This can be done by the security service property 'write Successful Logon Statistics'. Please have in mind that this functionality is disabled by default as it is causing performance issues. For more information see SAP Note 982741.
Best regards,
Nikolay