var customSearchControl = new google.search.CustomSearchControl( ...
This quick example specifies sorting by estimated dates for all searches. This example doesn't provide a button or menu to select sorting by date vs. relevance.
The sorting mode specified in this example uses approximately "estimated" page dates, which are not always accurate. Consequently you may generally expect some inaccuracies in the sorting order.
With further study of documentation and further work, it is also possible to sort by publication dates that can be specified in "structured data" (such as meta-tags, PageMaps, etc.) which can be embedded in (or associated with) pages.