The VF page will be injected as an iframe inside the account detail page and when you click the date link, the calender element is displayed fully but it cannot pop out of the iframe boundry. You will need to increase the height of the inject VF page to see the full view of the calendar.

The VF page will be injected as an iframe inside the account detail page and when you click the date link, the calender element is displayed fully but it cannot pop out of the iframe boundry and that is the reason it appears as cut. You will need to increase the height of the inject VF page or make the scroll available (settings on the page layout) to see the full view of the calendar.