Yes, you can log hours for past dates only for the active sprints in your project. You cannot log hours for the upcoming and completed sprints. Logging hours is based on the sprint's start and end date. Say for example, your sprint starts on 01/01/2018 and ends on 01/15/2018, you can log hours from January 1st to 15th. If you are in the middle of the sprint (01/09/2018), you can log hours for the past dates only till 01/01/2018.