They Witch's clock doesn't show the exact time for the next item. Instead, the clock show's an approximate time. If you go there right after the item change, you will see a message "There is more than seven hours to next stage". If you go when it's almost time fpr the next item, you will see a message "There is less than an hour to next stage".
The first item appears at server save, and the next ones is in 8 hours. So, the best way to know when you'll need to use the next item is to count 8 hours from the server save.
Each day there will be 3 items. First at server save, second 8 hours after server save and third 8 hours after the second item (or 16 hours after server save)