You can read about the auditing options that exist under HPUX in
this document at docs.hp.com. Follow the document to:
-->Administering a System: Managing System Security
----->Trusted System Security
When you built your system, did you enable the "trusted" option?
Once auditing is enabled, user login and logout times are one of the default actions that are recorded. The following are the event types which default enabled for auditing:
admin - logs all administrative and privileged events
login - logs all system login and logout attempts
moddac - logs all modifications of object access controls
The referenced document walks you through setup and use of the audit functions in HPUX. Fair warning, there is an overhead cost associated with running HPUX/Trusted.
Cheers,
Keith