apply_filters( 'learndash_notifications_user_login_track_unit' )
If the last sent time is larger than login time, that mean the notification already sent, we use the last sent time for the loop check else we use the last_login
apply_filters( 'learndash_notifications_user_login_track_unit' )
If the last sent time is larger than login time, that mean the notification already sent, we use the last sent time for the loop check else we use the last_login