How to update the User obj when the Session expiry time changes

Question

trigger AvailabilityStatusUpdate on User (before Update) {    
    DateTime lastModifiedDate;
    Integer noOfSecondsValid;

    for( User userObj : Trigger.New) {           
        LIST<AuthSession> session = [ SELECT LastModifiedDate, NumSecondsValid
                                     FROM AuthSession WHERE UsersId = : userObj.Id] ;    

        for(AuthSession sessionObj : session) {
            noOfSecondsValid = sessionObj.NumSecondsValid;
            lastModifiedDate = sessionObj.LastModifiedDate;
        }
        userObj.SessionExpiryTime__c = lastModifiedDate.addSeconds( noOfSecondsValid ); 

        if( userObj.SessionExpiryTime__c > System.now())
            userObj.AvailabilityStatus__c = 'Available';

        List<AggregateResult> loginHistoryObj = [SELECT MAX(LoginTime) FROM LoginHistory 
                                                 WHERE UserId = : userObj.Id GROUP BY UserId];

        DateTime loginDateTime = (DateTime)loginHistoryObj[0].get('expr0');
        Date loginDate = loginDatetime.date();
        if( logindate != (userObj.TimeNow__c).date())
            userObj.AvailabilityStatus__c = 'Absent';
    } 
}

I have used the above given piece of code to write a trigger. However, I want it to fire and update the USer Obj as and when the Session expiry Time changes. I am unable to fire it.

Firing the given query I get AvailabilityStatus and SessionExpiryTime blank.

SELECT username, id, AvailabilityStatus__c,CaseCounter__c, TimeNow__c, SessionExpiryTime__c from user

Please suggest.