I want to get the date when the Splunk admin credential got changed, is there any way to get it?
Unfortunately there is no such information field in the REST API for users, and the timestamp of the passwd file is updated when any user is changed.
The best option I could find is to search the _audit index for the "change_own_password" or "password change" actions. Hopefully the last password change of the admin account occurred during your data retention period for this index.
index=_audit user=admin action=*password*