MediaWiki:Apihelp-userrights-param-expiry
From RFF Wiki
Expiry timestamps. May be relative (e.g. 5 months or 2 weeks) or absolute (e.g. 2014-09-18T12:34:56Z). If only one timestamp is set, it will be used for all groups passed to the $1add parameter. Use infinite, indefinite, infinity, or never for a never-expiring user group.
