If you create a Calendar for YouTube for the time range 6:00pm – 7:00pm, then YouTube will only be available during that time range. If you are watching a YouTube video before 7:00pm, then you’ll be able to watch the video past 7:00pm, for a few more minutes. That is because of content buffering on the device. The video will be stopped after the buffered contents are exhausted.
What you can do with the router is you can set a schedule with the Calendar tool. What’s hard to do is to limit the total time on a web site without a schedule. Because of background network activities, it is hard to figure out exactly how long a user has been active on a certain web site. Therefore, the router does not support setting a total time limit for a web site without creating a schedule.