When you subscribe to a calendar you enable your calendar program to periodically check-in with the website and download the latest information. This will automatically "sync" with the Web calendar and you won't have to reupload information.
When you download the information, you are retrieving a copy of the calendar as it currently exists on the website. Our calendars change. Events are cancelled, times change, new events are added so your calendar will get become out-of-date. You will need to periodically return to this page and re-download the data for your calendar.