A DateField in a TabbedPanel may not be displayed if its not the active panel. However it will be included in the $jsImports which will cause a JS initialization error which will result in a Calender Alert message being displayed.
There are a number of potential fixes for this:
1 - remove Alert from Calendar.js, this is the quick and dirty solution, which leaves used JS code and imports in the page
2 - inline the JS initialization code in the page HTML for the DateField, has some performance issues with page rendering
3 - improve the logic for including JS, enabling the parent control/container to decide what is included.